Signing let's you prove what you did write, it doesn't let you prove what you did not write, since all the signed emails in the world don't prove the nonexistence of unsigned emails from the same sender. Now, if you have a regular practice of signing emails, one without a valid signature that purports to be from you is pretty strong evidence that either you didn't send it or you anticipated the need for future repudiation, but it doesn't itself prove anything about which of those applies.

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?

I've just switched to Postbox. It is a polished version of T-bird. And works with Enigmail to integrate with gpg.

You can't prove what you did not write.