Have you actually used this functionality Steve? To be quite frank it sucks. Sorry. The UI doesn't remember it, nor display it's active. At least with a different identity it's obvious that it's active.since our existing Alt-ID facility fully supports compartmentalization.
The idea with having, two identities, say "Paul" and "NaughtyPaul" is that I can pick the one for what I am doing (or about to do.) Then I go to the site I want to use it with and it "just werks." I don't need to keep entering some other identifier (beyond the Password/Quick Pass) to make sure I don't confuse myself ("I was sure I had an account at naughtysite.xxx oh well must have been deleted, let's create a new one".)
Okay, this horse is well and truly dead... I'll stop beating it.best argument you've offered so far is the idea of a third part giving someone an identity for them to use and share