From what I understand, it's most sought out by rogues. I can kind of see why, but then again I haven't leveled a rogue up to 30, so I don't really know what the norm is for the rogue gear around that level.
But yeah, some people are definitely obsessed with rare gear. I don't see the point, really, the only character that I have loaded with blue crap is my DK, and the only reason most of his stuff is still blue is because most of the improved green armor that I've picked up does the opposite of what I want to do (i.e. trading in +30 strength for an unnecessary +30 intellect, or buffing casting post).
Ah well, it gives jerks like me a good way to make fast money on the AH (that, and taking advantage of the idiots who sell ore incredibly cheaply).