Never in hundreds of trail nights, actually 1000+ trail nights, have I ever used one and never had tent-floor problems. Extra weight and cost for practically zero benefit. I suppose if I always camped in sharp-rocky areas I might partake in a polycro piece of plastic, lighter and cheap, like this:

http://gossamergear.com/polycryo-gro...th-medium.html

(I use this for my zpacks hexamid UL tent floor, above the bug screen, 2.5 ounces for the floor, amazingly tough but UL plastic)