I don't know whose brilliant idea it was to put a plastic liner at the bottom of kitty's litter boxes but I know for sure it wasn't a cat!
Cats HATE the liners. The litter box is supposed to resemble the sand/dirt where cats in the wild bury their waste. I can assure you there are no plastic liners in the wild.
Cat's like to use their claws to help bury their urine and feces so that predators can't track them in the wild. It works very well. When we domesticated cats and brought them into our homes, we created litter boxes so that they could do the same thing indoors. Although they do not have to worry about predators, ( in most cases), they still need to bury their waste. When liners are put into the boxes the cat's get their nails caught in them as well as the fact they do not like the feel of the plastic...it is unnatural.
I imagine the Genius who invented the liners was thinking about making litter box cleaning much easier for the owner. Not a bad concept really, except it does not work. As a pet sitter who has looked after hundreds of cats and cleaned hundreds of litter boxes, I can assure you that the ones with liners are actually more difficult to clean. The waste often sticks to the liner making it hard to remove and 99% of the time the cat's nails have torn through the liners and there is all kinds of litter and waste in the bottom of the box where the liner has been ripped. This means that you still have to throw out the litter within the ripped liner trying to make sure you don't spill any before you get to the garbage AND, you still have to wash out the litter box regularly with a mild soap before re-filling. Trust me, it is more work and really...the cats HATE it. Bottomline...Ger Rid of the Liner!!!