This is a very interesting debate! I hadn't thought of it much.
Personally, I don't like the idea of dual spec. The only other MMO I have played was FFXIV, and in that game, you could do everything on one character. I quickly learned that no matter how many nukes and DoT's I had, people always wanted me to come as a healer because I also had access to all of the healing abilities. I can see something similar happening here -- I plan to level up a Consular Sage to be DPS, and I don't want the possibility of being always asked to heal when I just want to throw rocks at stuff (I actually like healing, but that is what my Smuggler is for).
And yes, I know, "just get a guild" and "if you play with friends they won't mind". I'm not worried about this being an issue for me usually, but I don't want to be a total recluse that only plays with people I already know, y'know?
That said, I do see both sides of this discussion. I guess I just figure that if people want to play different roles, they can play different characters. The variety of roles available per class just seems like a nice way to let people pick the class that appeals to them story/looks/style-wise instead of it being solely based on the role you want to play. Just my 2 cents. ^^