That would be like having two separate login systems on the site. You can use one set of username/password to post in some places, and a separate set to post in other places. Additionally, the two systems would not effectively be able to work together. (Avatar, signature, etc. in one, Facebook account required in another, etc) Members who choose to block comments by a certain profile by setting up their ignore list wouldn't work across the whole site. Moderators wouldn't be able to delete comments because they wouldn't be tied to DaniWeb's user profiles, etc. Mods wouldn't be able to issue infractions, etc.
I guess there is the 'Register with Facebook' system that allows new members to bypass the registration form by just logging in for the first time with their Facebook info, and a new member profile is automatically created for them.