Try and find, for instance, an unambiguous description of how to derive "part numbers" for FETCH BODY[4.2.1] from the insane pseudo-Lisp you get from FETCH BODYSTRUCTURE.

no way to search in multiple folders

no way to change a message's title

Okay, I realize that you're not saying POP is better, but it is the only current alternative, and, as such, it reminds me of when Microsoft, trying to spread some FUD about Sun E10k machines said:

System boards that are hosting non-pageable kernel data structures cannot be removed from a domain without interrupting service. The Solaris operating system has to undertake a special "quiesce," or suspend, operation while the critical pages are migrated to another board.

So their machine can do remarkable things that yours can't even begin to think about doing, and you're complaining that one has to expend some effort to get it to work or that it has some limitations?

I'll admit that parsing BODYSTRUCTUREs is non-trivial at best, but POP has nothing of the sort. Okay; I think you can get just the headers of the message, or just the first n bytes, but nothing intelligent.

It's not exactly hard to send the same search to multiple folders, and POP doesn't even have the concept of multiple folders.

I'd say that changing a message's title is beyond the scope of any mail client. However, you could rewrite the message and push it back to the IMAP server, which I'm pretty sure you can't do with POP.