Custom bikes shine when your preferences stray from the mainstream. Perhaps you want some flex in your frame. Maybe you want to run wide tires with fenders. Maybe sometimes you'll take your bike on a tour. Or maybe you want integrated lighting so you don't have to worry about recharging batteries. Or perhaps you want all of the above. Production frames cannot offer this because these are niche preferences. I think fit is only a small aspect of the custom market. I'm willing to bet most people who get custom frames do it because their vision of a great bike is not met by the production options.