Why do people always think that this is somehow relevant to discussion? Everyday more and more people use this word, intending it as some sort of backhand slam. Are you holding someone at gunpoint, demanding that someone recant their opinion? No? Then why are you stating the obvious, which is that everbody has opinions? If you simply wish to state you're opinion without discussion, without resistance, why participate in discussion? Doesnt that defeat the purpose? Okay you have an opinion... and everything I said is opinion... what now? Do we just stop discussion?
I think part of what started this whole trend was when the whole "difference is strength" and "diversity is beauty" thing became a standard in american culture.
While opinions are much harder to prove than fact, there is a point, where so much overwhelming evidence can be brought to bear that an opinion can be basically true.
I especially can't stand it when people use "in my opinion" before every sentence in an attempt to be more politically correct.