Ah, well if I was A I'd recognize B's argument as dishonestly fallacious and would most likely be turned away from his cause. It seems like it could definitely make for effective rhetoric though in different scenarios, with more subtle cases, and with different people. However, I don't think Socrates would approve :)
Yeah, it seems pretty similar to the regular old Socratic Method to me. Except classically I think the Socratic Method was used more to reject a "stop sign" claim and provoke more thought than to make a positive claim. You know, Socrates and his whole "I don't know anything."
Also, the libertarianism example strikes me as a non sequitor: it simply does not follow that if you support drug legalization you support libertarianism.
This doesn't really bother me. Philosophers' expertise is not in making specific moral judgements, but in making arguments and counterarguments. I think that is a useful skill that collectively gets us closer to the truth.