One of the things that erks me the about the app stores model as a developer is it seems to have destroyed my ability to create an app and share it with a small group of people, as Chadk said, I may not want to release it into the wild as I don't want to have to support it for a large group of people.
Maybe I've missed something?
How do I justify spending time on WP7? Best answer for me is I want to write some private apps for my phone, but I don't want to have to buy a mac to do it. Everyone will have have their own answer; for some it will make sense, others it won't. Just look at want you want to achieve and make the best decision based on the information you have to hand. For my work, it's looks like we won't have an option but to write the same app in iOS, WP7, Android, and maybe WM6.X if we ever go that route.