Each breed is to be bred to exude "breed type".
To me, these dogs, while impressive in some regards and on their own merits, do not exude MASTIFF breed type. When the casual observer sees your new dog asks themselves, "I wonder when they got a Saint Bernard?", then your dog does not exude MASTIFF breed type.
I'm quite concerned with BYB-types focusing on "unique" or "rare" color patterns. If left unchecked, "rare piebald mastiffs" will be sold by BYB's (with no testing) for $10,000 ... while the litter of pups out of championed and health-tested parents sold by the seasoned breeder on the other side of town will go for $2,500 ...
...which will then lead to other breeders lying about their pedigrees and simply crossing a saint with a Mastiff to get a load of piebald running around that they can slap a huge price tag on...
Why do I think this? I saw it first hand when looking for a miniature dachshund puppy for my daughter seven years ago ... dozens of breeders were chasing the "rare isabella piebald" color pattern ... and any breeder lucky enough to put this pup on the ground would get $2000+ for it while the rest of the pups in the litter would sell for $300-500. How many miniature dachshunds were bred by unscrupulous breeders in an attempt to win the equivalent of the "puppy lottery"? Dozens? Hundreds Thousands? I don't know ... but it was too many.
I'd hate to see the same thing happen in my beloved breed.
So, how exactly does pursuing / promoting piebald mastiffs benefit the breed?
Aren't all breeding decisions to be made to first benefit the breed?
Besides, rescue will be totally overwhelmed with the dogs bred by breeders who want to capitalize on "rare piebald mastiffs".
Are they cute? Yes...because all puppies are cute.
Are they worth being loved? Yes...because all dogs are worth being loved.
Are they worth being bred and perpetuated in the gene pool? I doubt it. Until shown otherwise, I believe the risks outweigh the perceived "genetic diversity" benefits (as I consider those claims to be somewhat (or more than somewhat) dubious).