In the back of my truck, I routinely have:

  Gas cans
  Used oil containers
  Lawn and tree equipment
  Gravel
  Sand
  Soil
  Mulch
  Trash/refuse/recycling
  Mountain bikes

While a floor liner would certainly help, all that stuff is still going to destroy the inside of a family vehicle pretty quickly. And taking precautions to prevent that from happening means being extremely diligent about cleanup, whereas I don't care one bit if the bed of my pickup truck is dirty.