In a roundabout sort of way, I buy off of the rack.  After being fit for my latest set of irons, the standard clubs worked the best for me.  That being said, I would still get fit every time.  There's no charge and I really like getting the opinions from the guys at the shop I normally purchase from.

I think with the advancements in fitting, you are doing yourself a big disservice if you don't get fit by a PGA professional.

Another benefit to getting properly fit is gap fitting.  What that does is makes sure you have a good transition between woods and irons and irons and wedges. Many people have big distance gaps in those transitions which hurt their game

the only time fitting is truely a must is when you can repeast your swing consistantly and not before doing swing changes and buying off the rack is perfectly fine for a beginner but if your shooting low a fit might help some inconsistant habits that may be caused by an a wrong shaft or maybe a wrong lie angle

David I would say even beginners would benefit from properly fit clubs.  I have given lessons to many people new to the game who have clubs that are horrible for them and are doing them more harm than good. Clubs can be too short,  too long, shafts can be a poor fit, etc.  All of those factors will contribute to more bad shots and then the golfer will get more frustrated. 

I have always bought off the rack. With that said I just started playing golf again this year, the last time I played was in high school 2003. In '03 I finished out the season with a 1 handicap with rack clubs. However, now that I have decided to start golfing again I am taking the technical approach as well. I am in the process of purchasing the MB 710's. I do feel that getting fitted for my irons might help me shave off a stroke or two that I could not back in the day. Will it fail for me? Who knows. I do believe though that there must be some reason the pro's get fitted to the T.