When I ended up casually using PGP for personal email, it was for encryption when emailing with some friends at other schools, expressly because not everyone at one site or the other was trusted not to snoop on other people's emails. Not that PGP alone would've been enough defense against a determined and knowledgeable systems person, but it did presumably offer some privacy (and also, just the principle of it).

Then I fell out of the habit, so I'm curious in what kinds of scenarios you envision in which your habitual use of email signatures would help?