Would be beneficial for those who's signatures have information, and a possible nuisance for those with sigs that you couldn't care less for. As a sort of compromise, I'd like to be able to see someone's sig when I click on their profile. The information's already there so it wouldn't take a huge paradigm shift.
This way, you don't have to see the 10 line "this is my peni...er, system" sigs and are one click away from the ones that have more value to you. Given that the PM interface was recently given the valuable ability to click directly into someone's profile, I see nuthin' but good stuff.
Not to mention that if sigs were displayed in all PM's, the PM history view for a particular user could conceivably be a nightmare.