Each heat cycle cures the rubber a little bit more, so the tyre hardens with every heating it gets. Less grip over time. This is why new rubber always feels so good (to me anyway!).
I used some Sportecs that had done about 3 trackdays and about 800 road miles - loads of tread left - they let go without warning and spat me off at a very hot track day at Donington - the surface rubber had smeared when I checked them afterwards - as if the top layer had liquified!
However testers have commented that Sportecs go off quite quickly and it was one of the hottest days of 2003, but the same will be true of all tyres to different degrees.
Threaded Mode