Digital certificates is also a very good point. One of the issues in SL in particular though is that people often want to maintain their anonymity and a licence has to be with a real person at the end of the day. I guess (though not at all sure) that the licence agreement can be worded to tie to a person even if not named, although enforcing a breach would be impossible then. Having said that, avatar persona are often more "valuable" to a person as a member of communities that their actual real identity.