I don't have a backpack style bag, but I was just thinking about this.
Aside with being a huge fad (it is, don't lie) they are more comfortable than a bag with straps, and GRIP bags are near indestructible.
Still, I use a regular bag because I prefer them over a backpack. I think my bag with straps is plenty comfortable, it's not like I have terrible back pain after a day of golf. I can understand if maybe I had back issues, but I don't.
Also, I can carry more discs in a regular bag than a backpack (at least a GRIP bag). I carry 20-23 discs in my Innova bag, but my friends with GRIPS can barely carry 16-18.