I liken the "off topic" threads to the BS'ing, joke telling and general tomfoolery that occurs between songs at sessions. If sessions were nothing but music, music, music with no room for a little non-musical fun, I would probably stay home on Thursday nights. If the Mudcat was nothing but "on topic" musical threads, I would check in every now and then, but I certainly wouldn't spend hours here.
Off topic threads give us an opportunity to exercise our wit, which is something that musicians, singers and, particularly, songwriters need in abundance.