There is a lot more to a custom frame than the dimensions. For example: type of braze-ons, stiffness, durability, weight, style (level top tube, sloping), handling characteristics (fast/slow steering, low/high cornering clearance), ect.
For more info check the following link.
Bike Fit: Custom vs. Production
A frank discussion on custom vs. production bike fit.
I don't want to beat anybody over the head with my ideas of how a bike should or shouldn't fit or ride but the statement is often made that most people can be fitted on a production bike and don’t need a custom to achieve good fit. The basic premise being that correct dimensional fit can be achieved for an individual on a variety of bikes.
I have no problem with that statement as long as its meaning is understood: it means you can achieve a certain optimal distance for the rider to span on most any bicycle, but that is not "proper" fit on a bicycle. In the one of the QBP catalogs, there is a picture of one of the crew at about 9 or 10 years old straddling an old balloon tired mixte bike. The stem is reversed (pointing towards the saddle) and the saddle is down to the rails on the seat tube and extended forward. He looks pretty proud of his ride even though it’s so big he can barely straddle the step-through top tube. There you have it, proof of the concept that one can fit most any bike.
My problem is that I think the comment, "you can dimensionally fit on most any bike" is misleading if you don't think it through. The first rule of bike fit is to "do no harm." To achieve proper fit and maintain good handling you have to adjust for fit with an eye towards balance over the bike. To be confident on a bike in all types of conditions, you need to be balance fore and aft. It's that simple. The best of all worlds is that you can achieve proper fit while maintaining proper balance on the bike. Have I said that enough times yet? You can probably tell I’m moving into rant mode...
Here's the deal, I can't count the times I've seen people post about buying some bike based on how it “looks.” They give no real thought to seat tube angle, chainstay length, balance, cockpit combinations, or any number of other IMPORTANT issues regarding the purchase of a bike that meets your requirements regardless of what your desires for paint schemes or weld appearance or name brand cachet may be. If they're offering custom fit for free, get it! How can anyone who can afford to drop the kind of coin high performance bicycles demand need the bike so fast that they'll accept whatever is on the floor and still not know for sure it's what they need or even that it will fit?
I'm constantly amazed how someone will spend several thousand dollars on a bike and never give fit a second thought. Why are people who wouldn’t bat an eye at spending a $100 on a pair of tires, or spend hundreds on different stems or saddles or bars, or thousands on bikes so terribly reluctant to spend that same $100 to get fitted by a professional? It's not like you're going to a dentist to your wisdom teeth yanked or that long overdue physical, the guy who fits you is not going to drill your teeth or pull the glove over fingers the size of sequoias and ask you to lean over the table. He's going to help make you more comfortable and save you money. If I could give only one piece of advice to any cyclist it would be to get fitted professionally and have that fit session not be associated with a bike purchase.
One person told me that they could “fit anybody to any bike within a 1/10 of a millimeter” and “I can match fit to the degree that if you were blindfolded on a trainer with a stock bike and a custom - I could make the dimensional difference indiscernible.”
So what’s the point of that? I certainly agree it can be done, but I don't see how that is relevant to how a rider drapes himself over a bike. I can easily claim that I could fit a 5'4" person on a 60cm TT frame but that doesn't mean it will be right or even relevant to performance cycling. A big part of being properly fitted includes balance over the bike and handling. While I probably couldn't tell the difference between dimensionally identical bikes that arrived at those dimensions by different methods while riding them blindfolded on a trainer, I can also state I have no idea how riding any bike while blindfolded on a trainer feels. I've simply never had a cycling experience that required me to do so. We ride our bikes in a dynamic environment; our bikes are the interface between our bodies and the ground we travel over. Our bikes are not LifeCycles.
I guess my problem is that I can't see a way to separate proper "fit" from "ride" or "handling." A bike that "fits" well, yet handles like hell because of fit compromises is no better than a bike that doesn't “fit” at all. Obviously I'm biased, but I have a hard time understanding why anyone would compromise fit and handling when they can get a better product and better value by going custom than they could by buying a rack bike. A lot of folks spend 3K on a production frames alone. For that amount of money quite a number of custom builders can sell you full custom, full Record 10, Ouzo Pro fork and other quality parts for a bike ready to ride.
We all buy bikes because they have certain qualities we desire/need/prefer. It is not self indulgent to want both proper fit and ride anymore than it is to own multiple bikes or to spend a couple grand on a frame alone. If you can have both for equal or less money than a rack bike that may force you to compromise, what's your common sense tell you to do?
I'm not trying to harsh your mellow, I just want you to understand where I’m coming from.
Proper bike fit is more than just a top tube or a seat tube being long enough. It is also about balance and while you can adjust stem length, seatpost setback, and bar reach to fit most any frame, you need to do so with an eye to balance, the balance of your weight over the bike to achieve a certain center of gravity. You change one thing like setback to stretch or shorten your reach to the bars, you also affect your knee/pedal spindle relationship. Raise or lower your saddle, same thing. Raise your stem, shorten your reach, lower your stem, lengthen your reach. Can you do all these things and still fit on a production bike that is close to your size? In most cases, sure you can, as long as you approach it with an understanding of what you're doing. Adjust for fit, then achieve that fit balanced on the bike.