Up to now I've never felt the need. My last two bike purchases were both online, checking the geometries against my current set-up. One of those was a track bike; I dithered between getting a small or a medium, and finaly worked out I could have got exactly the same set-up with either, dialling in different positions with seat-post adjustment, stem size and spacers. (I doubt I'd honestly notice the difference in handling between the two set-ups.)
However I'm now considering getting a TT bike and there are a few reasons why I think it might be worth getting a minimal bike-fit service done, namely:
- still considering an online purchase but will be able to try out the bike "instore"
- similar "between sizes" conundrum, so may as well get it right the first time. Might as well spend £35 on the fit and get it right (you get a discount if you purchase the bike through the shop) than have to buy a different length stem afterwards.
- never even sat on a TT bike before, so need to get a feel for how aggressive a position I can get whilst still being comfortable.
So it depends on circumstances IMO. If you really haven't got a clue, and have a decent amount of money to spend on a bike purchase, it's got to be worth considering. However, I've got no idea how much benefit there is in getting an all singing and dancing £200 fit with lazers and videos and shit, over a £50 fit.