I believe they can be of help...provided you find someone that is well trained and educated.
I went to one last summer because I had gained lots of weight after my first year of university and it was discovered I had high cholesterol. I eat pretty healthy already but I thought I'd give it a shot since the apointment was free (because of OHIP). To tell you the truth, the hour or so I spent with the woman was a complete waste of time. I already knew everything she told me. She handed me some papers to read -one which was sponsered by Becel butter!! I flipped through the booklet by Becel and couldn't believe all the unhealthy foods that was being promoted as healthy. The booklet said to eat as much as 90grams of fat for men or 60 grams for woman...dude, there is no way I am about to eat that much FAT! The only thing that was interesting was the paper she gave me that showed a sample of a person's meal plan. But other than that, I was hoping she would help me with my chloresterol problem.

Needless to say I never went back...
I have tweaked my eating habits on my own, and have started to exercise as well. My chloresterol has gone down, and my midsection has lost 4 inches. I'm still hoping to lose about another 30lbs...
I know my run in with a dietician is probably different than most. I think they are a good idea though, especially if you feel you lack the knowledge concerning nutrition.