There is also: post something negative/semi-confidential about $work without implicating the work-place or revealing yourself; or ask something you really are supposed to know already without making yourself look like an idiot where your manager/rival can see it.
I like being able to say work is a drag right now because XYZ without actually trashing my work-place or letting hackers know where they can find an exploit or something I describe; I recently described my last work-place as having a cookie exploit which could delete the production DB; if I'd done that under my real name, anyone could read my resume and destroy the site. So "anonymous" posting lets me both vent and be responsible. This is 90% of why I keep myself anonymous which I do behind a username because I like my investment here. I just like it with a side of privacy. :)
That said, I also think catering to switching anonymous on and off is a mistake. I think it's nice that it's possible but it shouldn't be easy.
Sometimes the cost of associating oneself with a certain viewpoint is loss of employment. I could entirely understand someone distancing themselves from airing an unpopular viewpoint. If done responsibly, I see no problem with it. I still don't see that as so common a need that logging out and back in is a major pain from it. However, I refuse to blame a well-crafted tool for misuse of it by a person. Blame the person who misuses it, and not the maker of the tool.
Or maybe you just don't believe in pseudonymity when anonymity will suffice.
I find it interesting the three motivations you came up with seem to fall from the assumption of malice or jocular mendicity. Do you sell your straw men at a wholesale discount, or do I just pick them up as I need them?