This topic comes up again and again and again in terms of class balancing. I'll try to summarize my thoughts and opinions on some usual arguments about it. It is focussed mostly on dps classes, but can also be applied (to a lesser extend, and a bit re-furmulated) to other roles.
The difference between classes can make the difference between a kill and a wipe. The classes have different strengths, and that variance is good for the game. But the problem arises: When class x is awesome for dealing with situation y, and some others are bad for this situation, why not try to stack as many characters of class x for the boss? That is been done, mostly by hardcore guilds during progression. Those guilds are only a small amount of the raiding population, and an even smaller amount of the total WoW population. To erase that problem, an almost perfect class balancing in all situations has to be done, which is impossible. Thus, I don't see it as such a big problem compared to the benching in half casual guilds.
But, when it should be easily possible to kill the bosses, with the gear the average guild has when it gets there, why are still people getting benched?
1) They look to more hardcore guilds and imitate them.
Obviously that is one of the reasons, and in my opinion it's a very important one. How to change or solve that (without completely homogenizing classes)? I have absolutely no idea.
2) Differences in dps coming from the class are within a guild usually bigger than differences coming from the player.
This is one argument I hear over and over again: You're in a bad guild, you have to get better than the others there. Obviously you won't get benched because you're better than everyone else.
This is, in my opinion, completely wrong. The biggest difference between players is usually not if they can play their rotation. Sure, one plays it a bit better, another one plays it a bit worse. But that does, for most classes, not make a big difference. I'd say the margin within a guild is about 5% (with equal gear). The biggest difference is usually: How fast can they adapt to new mechanics / new combinations of already known mechanics? How fast can they master them, and while doing that play the damage rotation as good as possible? If on average your raid needs 30 tries for this to happen, there is no big difference if you are able to manage it in 10, 20 or 30 tries. The boss will not die until most of the raid has mastered this. And when most of the raids have mastered it, you probably do your 5% more, compared to what's possible with your class, than someone else, compared to what's possible with his/her class. But, only looking at single bosses, the differences between classes are usually a lot bigger than 5%.
Now the question arises: Are those differences really what stops you from killing the boss?
The answer: Depends. On an add-fight, 5% more DPS overall can mean a shitload more DPS on the boss. 5% can make the difference between the adds overwhelming the raid or being kept in check. Sha of Fear, the second phase, is probably the best example where a little higher DPS can make a phase much easier. Also on fights with tight enrage timers it makes a difference.
The next argument that is brought up often: You all should get better, with your gear there is no reason to bench anyone, the fight is trivial. Guild Z has done it with less gear some months ago.
In part, this is correct. If the whole guild would get better, everyone prepares for the boss, watches different videos, copies the char to the ptr to test the bosses there, ..., then the bosses would fall faster.
But: For many half-casual guilds, that is considered too much investment. The easier way: Sit out class x, doesn't make such a big difference, but the boss will die 5 tries earlier.
3) They lack other means to support the raid.
According to GC, not all classes need a raid-CD. I agree. But all classes need a way to support their raid, in addition to their main role. I'm not talking about respeccing in between the fights, I'm talking about support for the raid during a fight. Let's imagine the scenario: Class x and class y are comparably strong in terms of their roles. Class x has a strong defensive CD for the raid, and class y has a strong defensive CD for himself. Usually class y will get benched.
Note that also here not too much homogenization should come. But the differences in what classes bring to the raid, other than their standard role, are right now huge - there is almost no balancing in that regard. And thats mostly a result of the last patches / addons: Class A had something, then class B got something, then class C too, ... . And now, there are very few who bring almost nothing, or only very situational spells (e.g. 1 in every 15 bosses), and others have 2 or 3 things which are useful on every single boss.
An example how much that can do: The guild I was in when I played actively had 4 shadows in their roster (25). A guild where "bring the player, not the class - although the progress suffers from it" is used (which I personally like). At a time they were not able to kill Horridon, or Ji'kun, or Tortos, ... . But, because they had 4 shadows, Iron Qon was really easy because of Divine Star. If those would've been 4 (insert class y), they would also not have been able to kill that boss, most likely.
There are other points in my mind, but it's late and I can't write them in a conclusive and hopefully understandable way down now.