Not long ago my family was traveling western Kentucky on a four lane highway. I was driving. Up ahead in the distance I saw something large and black in the edge of the right lane. As we drew closer, it became clear what the thing was. It was a drop in bed liner that had gone flying from someone's truck.
Here's the problem. Plastic truck liners must fit very well. If they don't, you get wind under the liner. When that happens, the liner starts moving and vibrating. Then several things can result.
The liner can begin to crack at the hold-down bolts. Over time the cracks spread and the liner gets even looser. It could eventually go sailing away like the one I saw. Here's another problem.
A vibrating liner can also wear away at the truck bed paint, even a spray on bed liner. If the paint is worn away, that's a place for rust to start. Since it's underneath the liner, you may not notice until it's too late.
That's why you want a really good-fitting plastic liner. You don't want a liner that's moving with the wind.
What the drop in liners do especially well is protect from dents. The thick plastic spreads impact forces and all but eliminates dents. That's a fine thing since dents are way more trouble to deal with than scratches anyway...
The one piece plastic bed liners that are popular include several that you will find locally including the Pendaliner and the Duraliner brands. A bit different in design are the new liner from BedRug, the BedTred by BedRug, as well as the Dual Liner. Both a bit different and both tough plastic to protect paint as well as cargo.
The DualLiner truck bed liner in particular includes multiple pieces that for one makes it easier to ship to your house. What you get are tough plastic sides, front and tailgate covers. Then the bottom of the bed is covered by a matching rubber truck bed mat... It's worth a look.
Get one that fits right and you can't find a tougher and more durable bed lining. A thick drop in bed liner will stand almost any kind of hauling abuse.