When I bought my daughter's first cell phone it was prepaid. I spent more money on that then I did my own plan, so I added her to my AT&T plan. Now she just bought her own phone and started a plan of her own. I'm waiting for her to activate it, then I can remove the extra $100 a month (unlimited text, internet...etc...) from my plan.
I guess it all depends on how much you use the phone. My mother-in-law and husband use Boost mobile which is like a prepaid, the phone cuts off the day after the bill is due.