I prefer mutts myself, but I was raised my whole life with purebreds. We had German Shepherds, and almost anyone who has had one knows the heartache of the hip problem. Then problem is that since purebreds are inbred so much, you have defective recessive genes that pair up and become dominant, where in a mutt, they would more likely stay recessive. The other "problem" with purebreds is that they are so extreme......hard to explain so here's examples:
Pit Bull: Very protective and aggressive as a purebred, but mixed with say...Lab, not so much aggressive, and a bit more on the playful side.
Lab: Purebred....squirrelly, and very hyper, but mixed with say a pit bull, not so wild and a bit more protective.
I think that mixed breeds just aren't so extreme.