I've thought the same thing, too. But, as a one-time victim of identity theft, I understand the reluctance.
Moreover, I sometimes wonder if I only want that info so I can know more about my opponent's weaknesses when I disagree. For instance, on today's thread, if I knew where some people were from when they bash the US government/political system, I may very well counter-bash with specific examples of their own government's failings. That would not prove my side, now would it, and it would be - as you have been kind enough to point out before - ad hominem.
So, as you said, I put more faith when people have that info. On the other hand, maybe it's my own prejudgments that make me do that.