That brings me back. I used to use it with the mh mail client and exmh (which I think integrated with fetchmail). Despite exmh being written in tcl/tK, it was as nice to use as any "full fat" mail client I've used since.
The problem I eventually ran into back then was scalability. With the possibility of tens of thousands of emails, each with their own file, the mail directory could get really slow as the dir had to be rescanned for each sub-command. Mind you, that was in the days before the ext? filesystems had optimisations (automatic indexing or something) for huge directories like that. Even with the drawbacks, the maildir format still beat the alternative of a bunch of huge Inbox.bz files that needed to be decompressed twice when you were searching for something (once to find out which inbox file it was in, with no tools apart from zless) followed by a second decompress when you issue the command needed to extract the particular mail you want.
Of course, if I'd foreseen the need to index mailboxes before archiving I could totally have used something like glimpse on them instead of torturing myself with slow searches.
Nowadays, of course, all that seems like an anachronism when Google or Microsoft will happily index everything automatically. That's good, of course, but at what price?