The cheap insurance argument
A queen mattress runs a few hundred dollars at the bottom of the market and well over a thousand in the middle. A waterproof protector is a fraction of that. One spill pays for it.
There is a second argument that matters more than the first, and almost nobody mentions it at the point of sale. Nearly every mattress warranty is void once the mattress is stained. The protector is not just guarding the mattress. It is the only thing keeping the warranty alive.