I use comment signing as some kind of a multifactor.

I have my signing key saved on YubiKey, so it's pretty difficult that an attacker could gain access to it.

However, you can still commit through git web browser, and usually have a session for it open when working. If I slipped up and someone got to my PC while I have github open (or managed to steal my session cookies somehow - i.e a rubber ducky driveby), his options are:

• Commit without signing through SSH. I have ssh key password in my password manager that auto-locks after a minute, so that shouldn't happen, plus the commit wouldn't be signed since I have the key with me.
• Commit something though the browser - he can't sign it.
• Add SSH or a new signing key through the browser - I get immediately notified.

So, the end result should be that thanks to the signing mechanism, I should immediately know that something is wrong. Is it neccessary? Probably not, but I still think it's worth it, at least for me.

Now I'm wondering whether it wouldn't be better to have the ssh key on the Yubikey instead. Hmm. I did only discover commit signing later, and didn't have ybikey before, so it never occured to me.