The idea of a custom bike is a valid idea, but I have followed a number of custom bike stories from idea through construction and in a significant number of cases the resulting bike isn't exactly what the buyer had in mind. Sometimes the difference caused problems and sometimes it was not a problem, but a difference of expectations between the builder and the customer. In any case one nice thing about production bikes is you can validate how you feel about a specific model based on reviews, owners reports, test rides, etc.. and when you get the bike you can expect the product to be the same as the bikes you researched.
I bought an expensive touring frame recently - the cost was at the lower end of the custom frame spectrum. I considered a custom bike, but realized that since my body is normally proportioned that I preferred to have a nice production frame I could count on than a custom frame that might suit me a bit better, but also might disappoint me. In the end I've been happy with my choice.
Not to get too far off topic, you would have to pick a builder that has experience in building the kind of bike you want to ride. Going to a guy who builds full carbon 2kg racing bikes, to build a Chromo touring bike will probably be a bad experience. Going to a guy who has spent 25 years building nothing, but touring bikes, to build your touring rig and letting him guide the process, will be a good experience. As for frames, well, some people would be better on a custom frame, people who don't fit the 90th percentile height range for example, or people with an unusual build, may not fit a production frame well.
Hey if someone wants a Rivendell and they can afford it, then good for them, although for that kind of money, there are other choices, custom being one of them, which is why I put it out there.