The benefit to having a username field is that it can be unqiue to each person that visits your site. If you are only using first and last name then you may run into a situation where 2 or more people with the same name are trying to register. Of course, the users could just alter their names for the sake of registering but this would probably be more confusing than just coming up with a unique username in the 1st place. It is quite easy to add first_name and last_name fields though using the method I described.
For what I was doing I didn’t have to alter wp_usermeta so I don’t have any experience with that.
