It all depends on what you're looking for.
If you want nothing more than having the phattest loots on the server, you better play your character the way guilds want you to. I can do that just fine with my druid - hell I was pretty much responsible for us not wiping on our first successful Onyxia run.
Or if you're looking to be a top ranked PVP guild, you need to practice and make sure everyone knows their roles. Be it a healer, damage doer, movement control, flag runner, etc. This is a bit more flexible in classes, as some classes can excel in multiple roles.
I'm not sure what I want to do just yet with my priest. It is so fun to kill people with her right now, so I am enjoying the face-melter role. I just want to be feared when someone sees me and they are flagged. Be in in the world or BG, I want them to FEAR me when they see me. And I am already working on it
That role may change later on, or hell maybe I'll end up quitting the game for a while, who knows? My point is, you don't HAVE to play a certain way just because you are a certain class. That's bullshit and it's a part of the game I can't stand.