Kudu, I must disagree on this point (bolded). In order to even load the 870 the slide would have to be moved to the rear, a shell inserted upon the shell carrier, and the slide moved forward. That is one stroke of the mechanism. When ejecting the empty, the slide would have to be moved to the rear and the same process repeats itself, over and over again. And a well wear-polished set of slide rails is not a bad thing on an 870.
I agree with your assertion about wear on the mag feed in this situation, but what is to wear? Maybe the shell stops won't have been actuated, and maybe the inner mag tube won't show any scuffing.
That is a very minor consideration.
I have shot trap doubles with an 870 long ago, and while most trapshooters disdain that, using semi-autos or O/U's seem preferable to them.
If the 870 is slick it is easy enough to do if you are very well versed with the gun and are comfortable with the action.
Trap is basically about shooting rising targets in front of you, even with doubles. It is all about point shooting with very little leading the targets, and the second clay will be dropping so you must shoot under it.
Go to a skeet range and ask the puller to pull doubles while you are on station #8 facing the high house. You will definitely have to move the gun while racking in the second shell, with both eyes open to acquire the low house bird!
To me, trap is boring. Skeet is all about bird hunting practice.
Jim