Hot mop floors were popular before the invention of pvc and poly roll liners. These new liners are not subject to the cracking and breaking of tar. In the same manner that roofers have switched to rubber membrane roofs, good tilers have switched to membrane under tile. Every time you put hot water in the shower you have expansion and contraction. Tar lasts, but PVC and other plastic liners last longer. Still some of the older tile guys, very old, stick with what they learned and have not adapted.