If you were lowest dps and they took someone else that does more... then its understandable. If on the other hand you were doing as much as the others then they're just stupid.
Speaking from a 10 man point of view, you usually only have one of each class. You cannot compare a warlock to a elemental shaman, for instance. The point remains, if you're doing less than you could be, you're still being carried. Not doing your best because your average is still better than the rest is just a poor attempt at a very bad excuse. And the OP isn't doing his best in any way, judging by his armory.
Casual doesn't mean you're bad at raiding, or even slack at raiding. It just means you don't commit a large amount of your time into it. That actually means what little time you are raiding, needs to be pretty good.
Hardcore - Plays a lot of hours in the game. Casual - Plays a few hours of the game.
Nowhere does it mention being slack, bad or lazy is acceptable.