There are plenty of reasons to employ a trainer. Having him manage the horse is not one of them, unless the trainer is very sophisticated and has access to the necessary data for both his horses and all the ones it might run against.
Trainers are like general practitioner doctors-- they have to know a little about many things. But they don\'t shoe horses, or do all the things a vet does, and they are not equipped (in most cases) to do the kind of things I do when I advise a client, including advising on spacing. You would be shocked at how many decisions are involved-- which part of the country to run in (which involves, among other things, how good the horses in the division are, and what kind of races they write), which trainer to use, which rider, time between races (which means thinking a race or two ahead with stake horses), when to sell.
Horses coming off tops often look great, and fool their trainers. This happens all the time. The good ones know it, and listen.