Good question. A Fleckvieh/Angus cross should make a wonderful mama cow, which I would expect would milk better, and thus wean more pounds than the average Hereford/Angus. The Hereford/Angus may work better in areas with less forage (i.e. NV, WY, etc.) where cows have to travel more to get enough to eat. However in areas with more rainfall and thus more grass (i.e. NE, IA, KS, SD, etc.) I would expect the Fleck/Angus cow to be a better option.
Probably one thing that prevents more producer from using Fleckviehs in a breeding program is that there aren't as many producers of Flecks. And even in areas where there may be Fleck breeders, I would expect that many commercial producers aren't really familiar with them. I also wonder how many commercial producers don't realize that a Fleckvieh is simply a strain of Simmental.