In some armies such as the Dwarf and Wood Elf army, that expectation is they can do damage to any enemy they encounter. For other armies such as the Vampire Counts, Goblins (and probably Tomb Kings?), the expectation is the core units will not do any damage but the Special and Rare units will get the job done.
So lets get started.
Vampire Counts have line troops which are their basic infantry. As a non-VC player, it seems to me those are two-fold: Zombies and Skeletons.
No unit facing Zombies is going to be overly fearful. With an all-but-pointless WS of 1 and S of 2, they will hit just 33% of the time and wound even less than that; only against T3 troops will they even wound that often. Against troops with an "average" stat line of WS3, T3, they will need 9 attacks just to get one wound on average.
Of course, with their irrelevant initiative of 1, they will seldom have all their line troops attacking back since they are likely to be hit 66% of the time and, with a T of just 2, probably be wounded close to that often.
So they have an excellent chance of receiving damage every turn of close combat while only doing damage back in return once every four or five turns.
The Skeletons are slightly better with S and T of 3. So against other troops with an "average" stat line* (where "average" is defined as M4, WS 4, everything else but wounds, attacks and LD as a 3) they will still hit 33% of the time and be hit 66% of the time.
These are their core units, their basic block infantry. These units cannot be counted upon to actually inflict casualties do to their poor stat line. Their saving graces are their cheap costs, ability to re-spawn, and unbreakable nature.
Compare that to the famed Marauders. They have an average WS of 4, Average S/T of 3, and decent I of 4.
Against Zombies and Skeletons they are going to do some damage. Against others, such as Dwarf Warriors and/or Ogre Bulls, they are going to struggle. And against some it will depend on who has the hot dice...say, Repeater Crossbowmen.
Example: say I take 25 Marauders with Full command and give them Lt Armor, shield, and the Mark of Tzeentch to make them as rugged as possible. They start with a Static Combat Resolution score of +4 or +5 due to ranks, banner, and possibly outnumbering.
If I run into Zombies or Skeletons, I expect to win the combat more often than not even though they are likely to cancel most of my SCR. I figure to run my unit 5 wide so will more than likely hit 3 - 4 times per average turn and wound 1-2 zombies or Skeletons who are seldom going to save. Meanwhile, they will then have just 2 - 3 attacking back and hit perhaps once per turn which means I should only lose a Marauder once every 3 turns...less if my saves are working.
Conversely, if I run into Ogre Bulls, the numbers change. With my superior initiative I still look to strike first with 6 attacks (assuming a champion) and hit with 4. Now, however, I wound just 1.33 of them and I believe he saves one in 3. Furthermore, it takes 3 wounds to kill one and reduce his power to strike back. So it takes me 3 - 4 turns to reduce his combat power while he, conversely, is striking back 4 times, hitting 1.33 times, wounding 66% of those and reducing my save to 6+/6+. I figure to lose that combat if my SCR score does not force him to break.
Clearly, the Marauders then are a unit whose effectiveness is defined by what unit they encounter.
Contrast that with a unit such as the Chaos Warriors. They are still a Core unit but have WS5, S4, T4. That means against most CORE units, they will hit 66% of the time and wound 66% of the time. Meanwhile, they will only be hit 33% of the time and most core units will wound them either 50% of the time (for the very best enemies) and perhaps even just 33% of the time for the more common ones.
The Chaos Warriors, then, expect to win the majority of their close combats. It is an upset when they lose one (though I have lost every one I have been in but two....).
So what does all this have to do with anything?
My Marauders have faced:
Dark Elf Cold One Knights.
Dark Elf Repeater Bolt Thrower Crews.
Brettonian Pegasus Knights.
That is a much tougher than expected line-up. I have not yet gotten to go up against the softer enemies like the Goblins or Vampire Counts.
As a result, they have done two things consistently; died in droves and failed to do any damage.
I have yet to face the Goblin horde. I have yet to face the Vampire Counts.
Overall, they SHOULD encounter units they have at least a chance against. However, to this point they have only faced such a unit once (the Dark Elf Repeater Bolt Thrower Crew) and failed to wound in that instance...not a terrifyingly huge statistically upset since I had only about 4 guys attacking.
The Dark Elf Repeater Crossbowmen, meanwhile, have encountered the Skinks a couple times, the Vampire Counts at least once, Gnoblars, Marauders, and so forth; they have had targets that fall within the realm of what they can expect to damage.
Since I have not encountered units I can expect to damage but he has played a unit with a similar stat line that he has seen do damage, it is natural he would see the Marauders as a viable combat force whereas I, who due to a variety of factors, have only seen them battle units they have no reasonable expectation of damaging, see them as a worse than useless force...particularly in light of the following.
If I take a unit of Marauders that are intended to win a combat, they start adding up in points.
First off, I know I have to at least partially rely on static combat resolution. That means 5 wide by 4 deep, a minimum of 20 models. However, since they are so fragile and slow, I know they will take casualties so I want at least 5 additional models. So now I am up to 100 points, not a bad number. Next, I am going to take a Mark. One can make an argument for the Mark of Tzeentch to give them some defense against Magic, but I am more inclined to take the Mark of Nurgle which is better than an armor save.**
I am also going to take Light Armor and Shield if I don't know who I am facing since against Skinks, the Vampire Count army, and even Ogres I will have at least some save. I am now up about 160 points or so for a unit that desperately needs the charge to have a solid chance of doing damage against average opponents.
Compare that to a similar points value of Chaos Warriors. 10 Warriors with Full command, Nurgle, and Shield are about 200 points, or about 40 points more, give or take a nickel. However, they figure to do damage to any enemy they encounter and with T4 and a save of 3+/2+ will stand a lot of punishment.
So for a similar point total, the expectation is completely different. With the Marauders, their damage dealing potential depends on encountering a favorable match-up in opponents whereas with the Warriors they should always do some casualties.
So when I see two units with similar points values but one having the probability of hurting my opponent while the other is likely to just give my opponents some easy points, it is clear why I have such distaste for the Marauders.
The roll of screen is done better by the Warhounds. They both block line of sight, both are cheap, but the Warhounds have a M7 and are Fast Cavalry. The Warriors, meanwhile, can provide the same role as Marauders as a slow, immobile screening unit while still being worth getting into combat at a similar price. Their only disadvantage is starting 2 behind the Marauders in SCR.
So from the standpoint of looking at my own army list, other than a cheese use for Marauders (10 man units to grab table quarters), I just see other units that do the same job better for similar points. Kind of like taking Forsaken instead of Chaos Warriors :-)
On the bright side, that is further reason to take them. Units like them, the Chaos Spawn, maybe even the Forsaken will be appearing with more regularity. On the other hand, so will the Warshrine and/or Chariot :-)
So what are your expectations for Core Units? Are they capable of dealing damage or are they only good as screens, march blockers, Frenzy charge drawers, and so forth?
* A quick scan shows that in Core Units, about 3.7 is the average. Some armies such as Goblin Armies have 2s, most versions of Elf armies have 4s, Ogres have 3s, Bretonnian Man at Arms have 2s, and so forth. More armies have 4s than otherwise.
** An armor save forces the opponent to make no additional rolls. However, the Mark of Nurgle reduces their odds of striking by 17%, thus meaning they need to make more to-hit rolls to do the same number of wounds and thus have a smaller party of hits that still need to roll to wound. That is worded awkwardly so a quick number crunching.
10 rolls to hit needing 4s expect to hit 50% of the time, or 5 hits. If they wound 50% of the time, 10 shots will wound 2.5 times. Assuming the save is 6+ (Mark of Tzeentch), <1>
However, if it is 1 harder to hit, that same 17% alteration, the 10 shots now hit 3.33 times. Those 3.33 hits now need to hit 50% so approximately 1.8 wounds.