Just because Mailman does it doesn't mean it's a good idea. In fact, although I would not have expressed it in quite the way he did, I kind of agree with him. Storing passwords in cleartext is one thing if you've got three users; it's something else again if you've got hundreds of users, or thousands. It was on Perlmonks that I learned to build code that takes security into consideration first, code that runs under strictures and taint checking and so on and so forth. Furthermore, it was on Perlmonks that someone pointed out to me that a compromised password isn't just dangerous to the site in question, but potentially to user accounts on other sites, if users use the same passwords in multiple places.
I have always sort of implicitly assumed that Perlmonks used password hashing and per-user salt, because that's the way good programmers roll. I knew that some mistakes were made in the early formative years of the site, which have since been regretted (e.g., ISTR that someone specifically mentioned storing active Perl code in the database as an instance of this), but something as scary as clear-text passwords... I just sort of assumed that even if it had been that way years ago, it would have been long since corrected by now. I was very surprised to learn otherwise, and I consider the storage of passwords in cleartext to be totally out of character for the site.
If this had happened on slashdot, I would have just shrugged, changed my password, and gone about my life. But I expected more from Perlmonks.