Now that is a valid reason for wanting something by way of single identity. Finding that A N Other has nicked your personal id in one of the bits of the site must feel close to a form of violation (not to mention the confusion it causes). As someone who is big on integrity, I would find that horribly challenging (yes, those dozens of google references in some pretty weird places really are all me!)

I'd support a system that automatically refused the re-use of a username existing in one part of the system from being registered in another unless it was by the same person.

But I do still appreciate some value to being actually made to do something before I can access all areas just because I registered to a forum.