There are 3 big reasons to not enable it. They depend on how you use internet of course, but I'd never use full-automatic mode:

- Mozilla doesn't know what kind of link I'm using. I may be in another country on a pay-per-MB 3G, which they will not pay for. Updating a browser can actually create a really large bill, while all you want to do is open your email.

- Mozilla doesn't know what am I doing with my link right now. Maybe I'm almost saturating it right now with a Video call? I don't want them to start a not requested >100kB/s transfer in the middle of my call.

- Mozilla doesn't know how secure my environment is. I may be on a shared network on a security conference. Yeah... all kinds of things can happen there. That's why I'm not going to login anywhere from such place and will definitely not update my system.