Also, raiding is a group effort, and as such, playing the spec the group needs is the proper thing to do. I'm not sure why there is such a stigma attached to this for some people. If you want to play the spec *you* want to play regardless, then there are 5-mans and LFR, and normals after nerfs+gearing, which are so easy that it doesn't matter what you do. If you want to clear normal modes the first couple weeks, and move on to heroics, then you should be interested in what helps the success of the group as a whole, not your own personal preference.

For example, my raiding main is a hunter. We've been fortunate that all 3 specs are pretty close this expansion. However, if you want to be a hunter killing heroic Beth'tilac, then you need to be MM or SV, BM need not apply unless you already have an MM or SV hunter in your raid doing the hunter duties (yes, other classes can handle spiderlings but MM hunters do it best). If you want to kill many heroic modes in T13, you owe it to your raid to be SV. I don't care for BM personally, but if that was required to get a kill then I'd happily respec, reforge, and do my best as BM and be happy with the kill. As a pure dps class, it's my duty to my raid to know all three specs and bring the one that helps the raid the most.