I don't think you'll get too much of an argument from most people about mixed breed chickens. If you plan on showing your birds or breeding with a goal to preserve a heritage breed, then a mixed breed is completely worthless for those purposes. However, if your reason for having, and breeding, chickens is just to have some yard birds that lay eggs you can eat, any chicken will do for that regardless of whether they're purebred or mixed breed. The eggs all taste the same regardless of breed/parentage, and once a chicken is dispatched, plucked, and cleaned you can't really tell what breed they were anyway unless you knew to begin with. And chances are, the mixed breeds are going to be just as good, if not better in some cases, at it than the purebreds. I have an issue with people mixing breeds of domestic companion animals because there is a very real pet overpopulation problem in this country and I find it to be very irresponsible. But most livestock and production animals do not suffer from the same problem as any "unwanted" extras generally get eaten by somebody.