The purpose of the wear bars is to tell you that it is time for new tires, maybe safe to putt around town I guess. I think the limited amount of rubber still on the tire makes it susceptible to heat which also makes the rubber wear faster. Thinner tread also makes it more likely to be damaged by a sharp object and of course the reduced traction should be considered. Extending a tire from 10K to 11K is really not a lot of savings. I have not always felt this way and took chances more often when I was younger and wore then into the treads a few times.