I agree that a worn performance tire is no longer a performance tire but a non-performance tire with relatively high susceptibility to flats. Some folks who ride their performance tires well past the point where their designed performance advantage is gone would have better tire performance on average for the miles they ride if they bought middle grade tires and replaced them as soon as slight wear could be seen/detected. Let's say the lesser tire offered 80% of the "performance" of the racy tire when both were new but after 1,000 miles the racy tire's performance advantage was gone due to wear and the "lesser" tire was still "good as new".
This was more obvious to me on motorcycles than bicycles because their tires are wider and the effect on handling of a squared off tread made for such spooky handling that even I would notice.
