This implies you would also need to know if the user ID exists before you use it, which means a look up. (because it might be 'regenerated' and no longer valid)

This implies you would also need to know if the user ID exists before you use it, which means a look up. (because it might be 'regenerated' and no longer valid). If you take that one step further and just introduce disposable user ids, that point to a "real" user id, you've basically reinvented the session ID.