You wouldn't go to your primary care physician for a toothache.
You would go to a dentist because he would have the training, equipment, and daily practice to do the best job possible.
While you should a have good working relationship a primary Veterinarian there is nothing wrong with seeking out the best care possible when it comes to your horse's health.
I trust several different people to help me treat my animals.
There is a equine hospital near by I go to for colic and severe wounds (one of 3 major hospitals in Texas with reasonable prices with vets who deal with extreme cases on a daily basis). I see different Vet for osteopathy/ body work and holistic care. And yet another Vet who focuses exclusively on equine dentistry.
All are the best at what I use them for.
Because they do what they are known for all the time.
They don't float teeth once a week, they float 15+ horses a day.
You would choose a specialist for yourself. Why should your horse be any different?