It sucks away bandwidth. I don't think it's really a necessary feature. And on top of that, in pre-JKII release days, a lot of people had massive pics in their sigs, and it was a problem then. It would be a pain now with so many members and so many spammers/minor trolls going around.
And frankly I'd much rather have that search option than the ability to put pics in our sigs... it actually serves a purpose.