I have to agree with Janneke. There are some sport mixes that are very responsibly bred. Most often these dogs are bred for Flyball, and involve a herding breed mixed with a terrier.
The border-staffy mixes mix the drive of the Staffy with the intelligence and trainablility of the herding dogs, but also to mitigate a bit of the herding breed sensitivity. I think a team of 4 Borderstaffys holds the flyball record right now?
They are supposed to be great for what they are bred for. Super-fast, drivey, but with the "off-switch" that is missing in a lot of terriers. They supposedly breed pretty consistent, and the breeders heath test, and try to be responsible.
The border-jack is also really popular in flyball and agility. They are smart, super drivey and hyper.
As far as I know, neither of these mixes has been picked up by the BYB's and Puppy Mills. Neither of these mixes should go to non-sports homes.
Lots of mushers mix greyhound or whippet into their northern breeds to add some speed to their teams. Or they often choose to breed mixed breed dogs that are superior lead dogs or the like.
I see no problem with purpose-bred mixes, as long as their breeders are as responsible with their planning and placements as we would expect from any responsible purebred breeder.