There are problems with this:
1. If it forbids multiple profiles with the same name, some noob would enter his name as Fuzzlr or Sirr_Pig before they could and it would prevent them from being able to enter their correct name.
2. Almost everyone goes by their IGN anyways so it would kind of be pointless.
3. It would take a lot of staff work just for everyone to screw around with it.[