Just offering a different opinion here- mainly for anyone reading this thread who does not have a lot of experience with horses and an in depth understanding of horse health care. I absolutely agree that if you have experience and keep informed about new research, you can take care of all the normal stuff and just call out a vet in an emergency or when you need him/her. But I also think that having a relationship with a vet that knows you and knows your horse is a really helpful thing. I think it is like doctors for people- I probably could get away with not having a regular check-up or bloodwork and just go to a random doctor that I barely knew if I had a health problem, but I am very thankful that because I have insurance, I don't have to go that route.
I have only one horse and an excellent vet who specializes in horses and in dental work, so it makes sense for me to have him out regularly. Yes, I could do shots by myself, but a good vet will do so much more than just quickly look over your horse and give a few vaccinations. I actually have the vet do two check-ups a year. In the spring, he gets an annual exam with shots and bloodwork, and a fecal test. In the fall he gets an exam, fecal, and teeth, along with any other vaccinations or bloodwork that needs to be done. I don't have to guess about what kind of de-worming schedule I need, and whether it is working. I know what is going on metabolically, and can make better decisions about his diet and management. I can ask questions, even if they seem minor. And, this is a big thing, if I have a question about anything, I know that I can call my vet and get his advice whenever I need it. I know he'll show up if there's a problem, since I am a longtime client. I can trust his advice, which I think is invaluable.