I think it is carrier specific. They may be wondrous at the Fed, but at the Cat they are nothing but a boat anchor.
Case in point, ran a reefer that Chetjester drove for us two years ago and we used the lift gate I think twice. That was the last truck we had with one. Chet made good money but it wasn't because of the reefer or lift gate.
With the additional weight and maintenance, the nickel gets burned up in additional fuel costs because of the weight and several winter maintenance issues.
My guess is you would lose as many loads as you gain because you just cut the payload with the boat anchor on.
Or.........you now have to add a tag or second axle which is more weight to cart around, more tires, more taxes, more tolls, and the list goes on.
As to the original question regarding percentage vs mileage,, one can be as good as the other. Just depends on the bottom line.