A good trainer should be able to put together a better program than you can from just reading on the net. A trainer should evaluate what specific weaknesses you have and work on them, like posture problems, etc. Also of course, a trainer should know how to motivate you to keep you going.
In theory I agree - all great athletes had trainers. But as an amateur how do I find a good trainer and how do I know he is good? I have this friend of mine who decided to go to good life and hire a personal trainer. She was kind of puzzled by the program he made for her - very easy. It seemed like the trainer was kind of scared not to force her for legal reasons or something. I ended up making a program for her after discussing her goals and doing research on the net.