There isn't any proof that they overload the system when given all together. And we don't have proof that a single vaccination is all they ever need. When antibodies are created, they stay in circulation for a certain amount of time but just like with any other cell in the body they don't "live" forever and they breakdown or get used up and have to be replaced. If there is no repeat exposure, the body doesn't build new antibodies so it takes the body longer to respond to an infectious agent when it enters the body.
To try to make it a little more clear, think of antibodies as trained soldiers who have been educated on the type of enemy they are going to face---his strenghts and weaknesses, his tactics, etc. So when you vaccinate, the body produces these trained "soldiers" which then serve as a prepared force to fight a specific invading organism. But when these premade "soldiers" wear out, they aren't replaced except at need. So should a horse become exposed to an infectious agent the body has to identify the infectious agent, gear up and start producing new "trained soldiers"---this takes time (days to weeks to mount a good immune response)--which gives the invaders more of an opportunity to overwhelm the body's defenses and cause disease. When we give vaccines, the body treats it like an invasion and builds up it's trained forces which then circulate and provide an almost immediate ability to fight of the disease that you vaccinate against.
Licensed Veterinary Technician