The other day I listened to a pod cast by Veneretio at tankingtips.com. I really liked the dedication he show when it comes to being the Main Tank of his guild. He states that with the upcoming possibilities to dual spec, he will not have one spec for tanking and one for DPS. He will have two for tanking - since tanking is what he does. I can see all the arguments that for the good of the raid he should have one DPS spec and so on. But I still believe that he is on the right track here. Back in the days when I was raiding on a higher level and was acting MT I consciously never took any DPS gear until it was more or less forced upon me. I didn't want to DPS, I wanted to tank. And by not grabbing that DPS gear this was made perfectly clear to anyone. And as Veneretio say, people saw that I wanted to tank and thus gained confidence in that they wanted me to do exactly that too.
Even though I currently raid very casually I have been thinking of doing something similar with my Priest, i.e. having two healing specs, one holy and one discipline. This is not as "extreme" as having two protection specs since the difference is larger, but my guess is that many Priests will have one of their specs in shadow.
Veneretios attitude is inspiring, that if you know what you want, then you go that path and try to maximize all possibilities there are to be able to do it as good as possible.
Listen to the pod cast here.