I've used Mr. Tuffy with Schwalbe Marathon Green Guard tires since 2012. Lots of debris around where I live. Lots of debris getting caught in the tire.
The value of the tire liner is it is able to move around inside the tire. The embedded tire liner inherent to the tire is not.
Debris stuck in the tire can eventually grind thru the tire's embedded liner but once it encounters the tire liner that's where it stops.
I once ran over a roofing nail and it went thru the tire but the liner stopped it from puncturing the tube. I got off the bike pulled out the nail and kept riding. At that point I was sold on the liners.
After reading others experiences I took out the liners about 6 months ago. Got a flat within a month. Found a piece of metal embedded in my tire that ground its way thru to the tube. The liners are back on my tires.