Originally Posted by
Psyren
For the most part this is true, but some boss mechanics proc randomly and so having a spare/emergency ability (for when your class abilities are all on cooldown) can help, especially on a progression encounter where everyone's gear isn't quite up to snuff.
Also, some class deficiencies can be helped by racials. For example, Death Knights are weak when it comes to escape cooldowns (i.e. "run over here immediately.") The Worgen racial however gives them a speed boost, which can be the difference between your tank needing extra healing (or even a battle-rez) or not.