We'll have to investigate how to do this, and if we can come up with an acceptable, secure way, we'll gladly implement it as an option. The big problem I see deals with security and making sure that the user's Joomla account can't be compromised by someone logging into Facebook and stealing their account on your site. On a quick think-through, I think this should be ok as you can't easily update your email in Facebook to someone else's address (you have to verify you own it). If a user's FB or email account is compromised, there's not much we could do about it, and they could reset their Joomla user password anyways and 'steal' the account that way... just thinking.
Please feel free to come up with any other things to be aware of, and we'll look into how to best implement this on our end. I actually think it should be a pretty straight-forward change, but testing will be the key. I'm sure you'll be willing to help out with that part
This is also something we're starting to get more similar requests for as people are migrating to JFBConnect for bigger and bigger sites, so good to investigate now.