From my perspective, there are a couple of scenarios (that come to my mind as of now) where the need of having multiple accounts is justified and understandable
You missed a scenario that comes down to Bitcointalk being as free as possible, and offering free speech:
If you're hesitant to say something controversial because you don't want it to be associated with your name, please create an alt account and say it.
(but always with transparency regarding it)
As shown above, theymos disagrees.