Plastic Drop-In Liner Pros:
- Better distributes the force of impacts, better at protecting the bed from dents
- Less expensive than sprayed-in liner
- Easily removable
- Can be installed by owner
Plastic Drop-In Liner Cons:
- Slick surface allowing camper movement
- Not repairable
- Water tight which can let plywood camper bottom sit in 1” – 3” of standing water
- Very difficult to use with the HappiJac style front anchors
- Water, salt, sand, grit, etc still get under bed liner and cause damage
- Liner vibration will damage truck bed paint
- Liner reduces distance between wheel wells by 1” to 2”
- Too easily removable, with tailgate down, some can blow out
Sprayed-In Liner Pros:
- Can be repaired
- Rough surface helps limit camper movement
- Works well with the HappiJac style front anchors
- Seals the bed surface, water, salt, sand, grit, etc cannot get under bed liner and cause damage
- Bed drain holes are left open so rain drains from the truck bed
- Works well with heavy-duty rubber bed mat
Sprayed-In Liner Cons:
- Less affective at distributing the force of impacts and protecting the bed from dents
- More expensive than drop-in liners
- Not removable
- Cannot be installed by owner
Note: Apparently some drop-in plastic bed liners now come with a sprayed on non-skid surface on the top side of the liner to reduce sliding. Not sure if this is also on the underside of the liner to prevent the liner from sliding in the truck bed surface.
I think you missed the main "pro" of plastic liners. They distribute the force of impacts protecting the bed from dents and scratches. I have never had a spray in liner so I don't know if they are effective at stopping the dings and dents from shifting loads or objects. I have seen plenty of them all scratched and gouged, though.
My factory plastic liner is coated with a rubbery compound that makes it much more resistant to slip than a regular drop in liner is. It is still slick enough that I can slide and drag heavy stuff around without a lot of difficulty and I don't scratch the bed either. And, with a rubber mat on top, the camper goes nowhere.
I think a coated plastic liner works great for the truck camper owner.