I've learned a lot from trainers, being a nerd most of my life. Zero sports in high school where I think some guys pick some of this stuff up. Left to my own devices, there are exercises I enjoy pushing myself to do and exercises I ignore or just don't feel like doing. Lets just say if you focus on your pecs and not your back, bad things happen. Balanced development is important in some areas, especially in your legs if you run for cardio. Have to keep those knee cap bones stable.
My primary goal in investing in a trainer is for someone to step in and push me much further than I would accomplish on my own. Having that appointment forces me to get my own workouts in to make sure I'm not sliding backwards.
The one I'm working with now is aspiring to be a body builder, is young enough to be an offspring, and wears me out so much I have to skip a day. He's dating a nice woman in college and is comfortable with me talking about my gay problems. He's unique.
I do let them set up the exercise circuit which lets me disengage my brain and use him for the good company that he is. So, I try to avoid working with them more than one or two days a week or I become mentally dependent on someone telling me what to do.
When the cash flow gets diverted, I turn into a "ok, now what?"
24Hr fitness puts pictures of each trainer on the wall with their degrees, hobbies, and certifications. I haven't had a disappointment yet.