If I understand correctly bike fit is geared at finding the best dimensions for a bike to fit you, as such it is okay before you go and buy a bike but of limited use after you buy the bike.
Another approach is to get measured up on your existing bike and find out the optimum setup. I went to
http://irishfit.ie/
in Dun Laoghaire for a fitting on my road bike last year.
The first step is to check your flexibility and muscle usage. Then they use a turbo and video cameras linked up to a computer to measure your angles as you cycle, this highlights if you are cycling efficiently or if you have technique issues like knees moving laterally that can cause future injury.
The bike is adjusted to get as close to the theoretical best angles for your body type (ankle, knee, waist, shoulder, elbow) and goals (power, endurance etc.) and recommendations made for further adjustment if needed (crank length, stem etc).
You get a full report and a recommended set of stretches and exercises to target and issues highlighted during the session.
I did it last year, cost about â¬70 and took an hour, the differences were very noticeable on the road bike and gave a good idea of what proper position and setup felt like that was useful on the MTB and highlighted some muscle usage issues that were causing a series of injuries, so for me it was useful, if you are serious about your racing or just know that there is something not right about your setup it is probably worth doing, if only to find out that you just need to HTFU!