Waking up a 12 year old thread. I inspect my tires when I rotate them, and was catching a nail or other metal puncture once a year when commuting a lot of miles and driving through dirty streets and accident debris. High performance tires. Usually dug out a dozen popcorn kernel-sized rocks from the sipes.
I inherited a car several months ago and rotated tires yesterday. Only 13,000 on the odo, and may have been rotated once before.
Crazy amount of rocks trapped in the sipes. 200-300 per tire. Tires are geared more towards quiet touring. Most of the sipes are closed, so small rocks just get pressed into them and can't ever get out. Very few popcorn kernel-sized, and, mostly sharp-edged, not something mixed into the bitumen.
Oh, and it's a GM vehicle. Have to buy a $13 box to relearn the locations of the TPMS sensors.