As I'm sure some already know, I belong to a guild that focuses solely on raiding the 10-player content in World of Warcraft. Our guild has 11 players, and one of them was recruited (and he knew this before joining, and knows this is his role within the guild) solely as a bench player, to fill in when someone out of the regular 10 could not make it.
So far, this has worked incredibly well for us. A good number of us within the guild have been raiding together for over four years, and everyone within the guild knows his/her role, and knows what to expect out of every other player within the raid.
Most important of all for a successful raiding environment like this though, is that everyone in the guild is on the same page when it comes down to what exactly they want out of the raids we do as a guild.
We are all of the same mindset when it comes to raiding. This is key, because there are so many different mindsets amongst WoW players when it comes to raiding, and what each player considers "fun" for them.
WoW is a game, and therefore players should actually have fun while playing it, and should genuinely enjoy the time they spend logged in.
Over the span of the five years I have been playing this game, I have been a part of seven different guilds. I have been an officer in five of them, and the GM of three. Believe me when I tell you that nothing can cause more drama, issues, and tension than trying to raid with a raidforce that is made of players with different mindsets when it comes to raiding. This issue, and this issue alone can be at the heart of the majority of different types of loot drama/disputes, it is a major culprit of creating elitist cliques within guilds, and it can cause guilds to split and cause mass exoduses... simply because raiders within a guild want different things out of their time spent raiding.
If half the folks in your raid lean more toward the "hardcore" side of raiding, and the other half tend to lean toward the "casual" side of things... the two mindsets are likely to inevitably clash.
Some folks in your raid may not care about how many bosses they get down in a night, and some may become frustrated with anything less than a full clear.
Some may not feel that always making sure they have epic quality gems or high level enchants are important, and others may sacrifice their first-born child in order to afford the shards for a +63 spellpower enchant over the +50.
Can guilds made up of players with varying raiding mindsets function and succeed at raiding. Of this, I have no doubt. I just feel that it is much, MUCH easier when players have similar mindsets, and all want the same things out of their raids. If your raid is made up of raiders who want the same things, and are expecting the same things out of everyone else... you can avoid the clashing (when it comes to raids) altogether.
There is absolutely nothing wrong with wanting a relaxed raiding environment, nor is there anything wrong with wanting a more hardcore environment. Everyone is different in terms of what they consider "fun." I personally just feel that making sure you are raiding with like-minded players will ensure that everyone in the raid is having a good time... and not some having fun, and others pulling their hair out of frustration.
This is ultimately the reason why we choose to only raid 10-player content. Finding 10 like-minded players... is much easier than finding 25. We figured that while the gear may be better out of the 25's, the frustration of raiding with people that may not want the same things out of raiding wasn't worth it.
We'll take the few ilevel points hit to our gear in order to always have a good time.