I would only use iMessage if it had a setting to be the backup, not the preference. It will never make sense to cancel unlimited texts on my family plan for limited data on a per line basis. As long as iMessage uses my data plan, it's just too expensive. I want to use real SMS/MMS exclusively if I'm not on wifi, and even on wifi, I'd only want iMessage to kick in if I try to message an iPad or iPod that lacks SMS. But you can't set it up that way so it's useless.