Just depends I guess. I keep my NN bantams in their own pen because I dont want crosses, but my laying flock is a mixture. I didnt pick the birds for color really, I close them for egg color. I get brown, blue and dark brown eggs in my layer coop.
I like so many breeds, I'd have a terrible time choosing just one! In fact, I have a mixed batch arriving in June!
Aside from that, I love looking at the mixed flock wander about; they are as soothing as a fish tank, IMHO.
Also, as you said, different breeds do have different qualities.
Just as different tomatoes/tomato plants have slightly different qualities to recommend them, which is why I'm growing so many varieties again this year!
Mostly I do stay with my heavy breeds, though, like the Buff Orpingtons, Black Australorps, etc., though I do have some EE's. I have found that the mellower breeds tend to be happier with other mellow breeds in their midst. Still, I managed to find 10 breeds to fit the bill!
I have a lot of different breeds. When I first went crazy headfirst into this chicken addiction, I got lots of different breeds. Now that Im an old hand at this chicken business (3 months....ha!) Im finding Im narrowing my interests a bit. I have one big main coop where the main layers are growing up, but Ill have a few side pens to put a few breeders. The ones Ive decided not to pursue breeding on can stay in the main coop, lay eggs and be pretty. Theres always room for those.