I used a personal trainer when my pup was about 7 or 8 months old for on leash behavior and overall it was money well spent. I agree with DJEtzel regarding what a trainer does, Yes, I know there are various approaches and some training programs will take your dog and train it for you but I don't think that is what you are talking about. The trainer we hired basically taught us how to teach our dog...and what I liked was the trainer proved his methods by displaying them using our dog. I rather enjoy the fact that there are those out there who will broaden your horizons simply because they know a ton more about dog training than the average and above average owner.
How about teaching your dog to become proficient off leash? That is personally my goal and it seems to be a bit more work than on lead. Ultimately, I want to take my dog anywhere and everywhere I go, if allowed.