The tyres are worn down to a point they were never intended to run at, I don't know but the rubber compound could be completely different that far down, not to mention the risk of punctures.
Well, safer and better in the driver's mind until they fly too close to the sun and realize following the accident that there was a puncture or that the rubber delaminated off the belt during the commute. This happened fairly regularly at the track I worked at, though that was more from folks running their slicks too long.