I've always used cheap plastic sheeting for the same reasons Doug put up. Have used pieces of mattress plastic or paint drop sheet plastic, cut to slightly smaller than tent floor.
Always under tent, doesn't make any sense to put it inside tent.

I've found it very useful in protecting tent floors in gravelly sites, and handy also on rough cement if you ever pitch tent under a shelter, less abrasion for tent floor.
Seems to me that plastic sheeting is just as light or lighter as the more footprint things sold with tents that cost 30 bucks easily. Plus I never set up a tent with no tent body, mosquito less camping or other critters is not something I've experienced in summer camping.