Just about every horse has something they can pass genetically. Unlike HYPP, where you do not need 2 copies of the gene to have the physical traits, HERDA in recessive form is harmless. The only time it's an issue is when you breed 2 HERDA carriers to each other (and then it only is about 25% of the time). Why pass up on the opportunity to breed to some outstanding horses, just because you need to pony up (no pun intended) to have your mare tested before deciding?
It's similar to SCID in Arabians or LWOS in Pintos/Paints. If a horse is a carrier.. they are just like any other horse, just have 1 gene that you need to be careful when selecting a mate for them. I know of a few SCID carrier stallions that require mare owners supply a negative test when applying to get a breeding, and I'm sure most stallion owners with a Herda postive stallion would require that as well.
It's really, not that big of a deal, unless you don't want to test your mare. And if that is the case, I recommend not breeding at all.