I didn't find them helpful. Working out the best diet for me was something that needed trial and error, rather than the expertise of a dietician. Since different foods suit different people - even people with the same illness, Crohn's - they can't know what's best for you any more than you can yourself. There are some ideas that apply to quite a lot of people - e.g. eating little and often, experimenting with different amounts of fibre to deal with constipation or diarrhoea, increasing of decreasing the amount of calories you consume to gain or lose weight, but this information can easily be found in books or online. Also sometimes you may find that guidelines that are suitable for most people don't apply to you - for example, a lot of advice I've been given said that if you feel too full, by gradually increasing your portion size, you'll eventually be able to manage larger portions and feel hunger again. I tried this, but my hunger never came back, and my feelings of fullness seem to get worse.
I'm not exactly sure of the difference between a dietician and a nutritionist, although I believe in the UK at least, dieticians have qualifications based on more medical or scientific knowledge. Dieticians work for the NHS, whereas nutritionists don't as far as I'm aware. That would match my experiences - nutritionists seemed far more interested in alternative therapy. This was no good to me either. They (I tried seeing a couple of different nutritionists) told me I was allergic to various foods that actually gave me no problems, and wanted me eating "whole foods" full of fibre that upset my digestion and restricted me to a very limited diet.
Possibly a dietician (or a nutritionist) would be useful if you're unsure of the basics of a good diet and aren't able or don't want to research information youself. I think some people may really benefit from the motivation to improve their diet which a dietician could provide. But I didn't find them beneficial.