Having no liner is better in the long run than a plastic drop in. Truck bed floors and side walls are galvanized steel. Even if scratched to bear metal, the worst that will happen is a light haze of surface rust. I had no liner in my '07 F150 for 8 years and yes the paint inside the bed was almost completely scuffed up to a dull blue (factory paint color) and there were many deep scrapes and gouges, there was only a few spots of light surface rust on the really deep ones. It would rub right off with a rag.
The problem is that with a plastic liner moving and shaking around, even just a tiny bit, the paint gets worn away, but also then any moisture that gets trapped under there stays for a long time. It can't air-dry. The moisture pushes the galvanized steel beyond its corrosion resistance.
If you only plan to keep a truck a few years, than it might not really be a concern. I got a steal of a deal on a drop in so I popped it in there for now. In a year or so if I decide I'm keeping it for the long haul, I'll pop it out and get it sprayed.