I would welcome duel specs with open arms and a "What took you?" I was so torn upon realizing that my RP character Rialla could not be a sneaky sort of healer. Her real, in-character methods cannot be used ingame, and that... detracts from her character significantly, as the reasoning behind her methods runs a little deeper than most would realize and actually says quite a lot about her. As it is, the closest I can get is... DPS. That's it. Bleh.
But I think you should have to earn the chance to duel-spec, like, first you have to reach the sixth tier in one of you skill branches, or reach level 35, or something like that.
For those of you that are worried about duel-speced people having some sort of unfair advantage--which is the only remotely logical reason someone would protest--please consider. You have a set number of skill points. The skill trees were designed with that in mind. If you were to suddenly have a new skill tree, but no new skill points... do you see where I'm going with this? Jack of all trades, master of none; journeyman of two trades, master of neither. They would be no more powerful. They would, however, have the opportunity to be a lot more creative in their fighting style... and isn't that something Bioware promised us we'd have?