Advantages of Going Custom
Why custom? Because if you're going to lay out big bucks for a high-performance bike, settling for anything other than a frame designed for your body and your needs is counter-productive. The only motor that bike is going to have is you and if that motor is going to deliver efficient power over prolonged periods of time, it has to fit you correctly.
Production bikes, including the top-of-the-line models, are built in batches to predetermined measurements to lower manufacturing costs. Even the so-called 'professional' models, whose prices approach or even surpass those for handmade frames, are still based on standardized formulas. Custom frames on the other hand are designed and built expressly for individual riders. In other words, a rider's measurements (height, leg and arm lengths, etc.) dictate the frame's geometry (the lengths of the top tube, the seat tube, the chain stays, the various angles that join the tubes, handlebar stem reach and rise, etc.).
But there's far more involved in the creation of a made-to-order frame than just the geometry because in truth people have been measuring riders and building bike frames for them for so long that today geometry variances are minimal. What sets custom frames on an entirely different plane than production frames is tubing selection and finish work. Production frames are built with standard tubing groups to minimize production costs but there's a major problem inherent with that approach.
Take two riders, each the same height, same leg lengths, etc. In a store selling production frames, the two riders will receive exactly the same frame. The problem is that one of the riders weighs a good 20 kilos more than the other and has pedal crunching leg power that practically ripples the pavement when he goes into sprint mode. The second rider is lean like a whippet and a stunning climber with a leg speed that melts the grease in the bottom bracket bearings. The climber will probably think the new bike off the shelf is okay enough though heavier than he'd really like. The first rider, the guy with the herculean legs, will jump on the same bike and go into sprint mode to check out how the frame feels and end up walking back with his arms full of broken tubes. Why? Because production run frames including the so-called pro models are built for theoretical 'average riders'.
There are no 'average riders' in the world of handmade, custom designed frames. Every rider is unique which is why Eriksen frames aren't built with standard tube sets. Kent individually selects tubes for each part of the frame in order to best meet each rider's needs. In the above case, over-sized and possibly even thicker walled tubing might be used for the sprinter while the emphasis for the climber would be minimizing weight. The climber would no doubt find the sprinter's bike a tank on the hills while the sprinter wouldn't even touch the climber's bike for fear of twisting it into a pretzel.
Hand-built, made-to-order frames are also all about the little extras buyers want for their particular uses. Like spacing for bigger tires, attachment points for fenders and/or racks, a higher handlebar position for older riders, etc. Which is why if you're looking at popping for a high performance bike, you're way better off doing the full monty and ordering a frame built expressly for you and your needs. 
The price differential between that handmade, custom-designed frame and an off-the-shelf top-of-the-line bike from some big manufacturer is considerably less than you may think.
