I don't think apps should run in the background all the time. I generally approve of the idea of having one program running at once, with a few exceptions. (An ssh client comes to mind. It's a pain to realize you need a piece of information from another app and have to logout and back in again in order to retrieve it.)

I was really commenting on the fact that there's no service/daemon framework. Developing one of those would increase difficulty. Apple could even design it so that they weren't allowed to have UIs, so that people who wanted them would be forced to write two applications that communicated with each other.