Jimbo-
It seems unlikely to me that spammers would bother to abuse a web-posting interface. It would be a lot of work, and the one thing spammers like to avoid is work.
Perhaps. But have you ever heard the term "crapflooder"? There are other problems besides spammers, and I often use the term spam to refer to them as well. On my weblog, infoAnarchy, I regularly have to deal with people who think it's fun to post hundreds of nonsense comments. The same is possible on a BBS. It's not a big deal usually because you can delete the stuff. But you don't want it to be sent out to users on the mailing list first.
You're spoiled by wiki. Everything is easy to fix on wiki. But crapflooding of a mailing list by some banned asshole would be very annoying. The mailing list confirmation is at least some barrier to entry, where you can deny access to individuals easily or simply limit new signups in emergencies, and an equivalent barrier should exist for a BBS->list interface.
Regards,
Erik