If you're out on the tails of the bell curve as far as body size goes, then there's no doubt you can be better served by a custom frame. But in those cases, we're not talking about differences of a millimeter or two. I think in cases where "normal size" people buy custom, the ride and comfort they attribute to fit may actually be accounted for by the fact that they getting a frame tuned to their weight and riding style and often hand crafted from the best tubesets with no compromises for mass production.

It's likely a high-end off-the-rack frame in the same price range would ride just as nice. But when you reach the price range where custom becomes an option, the other advantages of being able to spec the exact configuration you want really sweeten the deal. When I ordered a custom CX frame, I was able to spec the tire clearance, cable routing, the exact braze-ons I wanted/didn't want, custom travel couplers (Ritchey top, S&S downtube), etc. I love the fact that I was able to get the exact frame I wanted with zero compromises. People who think fit is the only reason to go custom aren't seeing the whole picture.