Because a measurement system can't possibly account for the vast number of variables that ultimately contribute to the potential speed of a boat. For instance, how you calculate the speed improvement provided by a full lifting foil? What about a foil that provides partial lift and doesn't clear the hull from the water? How about a foil canted at 45 degrees vs. one canted at 20 degrees? How about a manufacture that got the incidence of a canted or curved foil correct vs. one that got it wrong? What about a rounded bow vs. a plumb bow or a full bow with high volume vs. a narrow bow with it's volume down low? An undercut stern? A high mounted trampoline vs. one with not enough height to clear waves?

What about the differences in performance between a strict SMOD class that hasn't seen a sail shape evolve in 15 years versus a box rule MMOD class that sees new and more efficient sail (and hull) shapes every year? Though the sail areas stay identical, we've seen this in the dichotomy of the N20 and the F18s as the F18s continue to make slight speed gains around the course due in part to advancement of their hull and sail shapes. The technical measurements of the boats remain practically the same.....I could go on but the existing measurement systems are very strict and unforgiving and lack the ability to refine their figures beyond a sheet of numbers and very complex formulas that do pretty well but are, at best, no more accurate than the performance based systems we have (they typically even carry one less decimal point).

That said, it does have advantages...but you ask for the weak points.