Back in the 70s and 80s, it was common to buy a new American car with a double-digit number of serious defects right off the line; Japanese cars had their initial success in the States when they were generally fine at purchase. Overall quality has been greatly improved since those terrible days, but it's still a useful measure. Nobody wants to have to keep taking their brand new car into the shop.

Also, if you wait until you get any substantial real-world numbers on TCO from owners at 150k-200k miles - collecting several years of data - any given car will likely have been redesigned and have a different quality profile. You can extrapolate by manufacturer, but given the extended timeframe it isn't all that helpful - or even more helpful than initial quality - when buying a brand new car.