BB Messenger is less about the service that's offered (threaded, always-on, instant messages) and more about the "exclusivity" of the service - a private, BB-only club. Whether you think it's silly or not, this is a very appealing feature to some people. I think Apple would win over a lot of BB users if they developed a similar, iPhone-only IM system.
I wouldn't mind seeing an iPhone Messenger myself as there's at least 6 other people where I work that have iPhones. Some of my friends might getting iPhones and my mom who has a Blackjack II wants an iPhone badly after playing with mine lol.