The important caveat is "assuming linear performance"



This is not the case in the real world.



The problems with using a single number (Texel or DPN or ISAF) is that a boats performance is not linear over the wind range that we sail at and so it won't be "fair" at some windspeeds. For example... Bruce used the I20 versus the Tornado. The F18HT class is extremely non linear in its performance. This boat owes time to the 20 footers in beufort 3 and under!



Since the goal of any handicap rating is to normalize the boats and allow the crew work to decide the outcomel PRO's must use the windspeed adjustments to ensure fair racing.



Distance races are problamatic. Most clubs use the strait DPN number because it may not be reasonable to assign a windspeed for the entire course.