Also, to "help" people decide wich spec to play, I always suggest "pick the spec you're the most comfortable with, unless you are familiar with all three".
When a new raid comes out, you have to deal with unknown encounter's mechanics WHILE dealing max DPS. Most people can only do only one of them, so if you're not top level you should try to maximize both. And the easiest way is to play your most experienced spec.
Progression raiding is NOT a good place to learn new class mechanics