Those that do are asking for a cylinder to start PreComp. At this point, the disk may make multiple passes to attempt to get valid data, or may even fail completely. From the figures above, we can see how a slight amount of Precompensation can insure long term stability. The disk that didn't employ PreComp was eventually unreadable. Of course, this would take time to happen, I don't have cold hard specs on how much drift will occur.
Few people realize what goes on behind the scenes during a disk read.
For one thing, each disk is programmed to retry failed seeks. If valid data still can't be obtained, an attempt will be made to use CRC information to reconstruct as much of the data as possible. Only when it is too late, and you've lost your data, will DOS inform you.