A lot of guys say you need it to stop ceilings from drooping, but I find if the framing is right it holds up just fine. It really speeds things up on smaller projects.
Actually, I read a study from a building science group that said back-blocking isn't always needed. They checked a bunch of houses and found ceilings stayed flat if the joists were spaced correctly. Like @gavinbaker said, good framing is key. It saves time and money on smaller jobs.