1. You play the optimal way and can do all the "advanced" endgame.
2. You play however you want but you're not gonna do all the endgame.
Will you be what they want you to be, or will you do whatever you want? Make your choice.
"Reminds me of a dog, you stare at it while it's playing outside... Running around in a circle, then stopping up, wagging with a wide "grin". Jumping around until you call for the dog, then it lowers its tail because it think it's done something bad... And then you throw the ball, seeing the tail go straight into the air in bliss! ^^" - Gehco Wishblade of Draenor EU
I agree with most here, this really only applies to raiding - anything else it really doesn't matter how you play - whether it is a 1k dps boost or not matters little in a dungeon or while questing. But, raids have raid leaders, other players are all depending on you to do your best - and research where research is needed.
But otherwise, play how you like - you are right it's just a game...