True, I would only use a trainer for something I needed them desperately for, but I like to take pride in my work. Also like you said, you don't know everything about horses, and neither does a trainer, yes, they may know more, but not everything.
I worked with a trainer last summer. I learned what I could from him, and am currently working on my own until I perfect what I learned. At that point, I would like to learn what I can from another trainer, and so on. :)