The holes are just like wear bars on conventional tires. You should be able to run them safely until they are gone - with the same tractability. The compound doesn't change as the tires wear down. It does change with age. The issue is uneven wear, which you have to start looking at when they are about 1/2 gone. Obviously if you have ply's showing, you gotta dump them. One of the reasons I run tubes. I could go ether way with my tires. It's just an added safety margin IMO.