Why cleartext digital signatures are dumb by AdamShand ...
> another way to look it is, if you're dilligent about signing emails, you > can prove that you didnt say something. It becomes impossible to forge > emails - the content at least.
No you can't, you can prove that it's *unlikely* that you sent it (maybe you were away from your normal computer and using a friends computer or a web mail account). All signatures really allow you to prove is that you did send the signed messages.
I would rather be able to plausibly deny that I sent any email message then implement a solution to be able to prove that I sent some subset of messages which have my name on it.
Besides which, Google is bad enough when it comes to researching past sins on the web, I don't feel that I need to provide cryptographic proof of it to the world. 
Now if you're encrypting the message as well, then that's different. Then it's worth signing your messages so that the recipient does indeed know that it's from a certain sender.