Hey Chickenboy, I'm no expert but it's my understanding that the main value in raising sustainable heritage breeds is for preservation purposes. According to the Livestock Conservancy there are 11 breeds that are currently threatened with extinction. So for those who can afford to do it, purchasing heritage breeds is a good thing.
As for mixed breeds, many people enthusiastically embrace the mutt chickens. (Note: I'd love to hear more on this if anyone out there disagrees). We're just starting our new flock after several years hiatus and if I recall correctly, we had several robust mixes last time and they were great. Eggs: check. Meat: check. I don't know if this helps, just tossing in my two cents. Enjoy your birds!