I have a number of vehicles with many hundreds of thousands of miles on them; including 7.3 powerstrokes. I've had enough oil analysis done on my cars and trucks to find that mos2 does seem to reduce iron wear on the same vehicles vs. not being treated. Seems to be more pronounced in the winter months looking at uoas. Also suspect an effect on injector oil poppet valve life specifically on 7.3 diesels; small sample but trucks that were dosed have significantly better injector life with regards to that particular failure point.
In a situation where maximum life from an engine is reqired, the engine will actually have a chance to achieve very high milage / hours and someone doesn't mind the added expense of mos2 at every oil change, mos2 may squeeze a little more life out of some engines and engine components. I started using it in some engines a number of years ago 20+ ? and run it in all of them now.
If I were contemplating using the additive, I wouldn't waste the money or effort if I wasn't planning to keep the car for more than 250k anyway.
I also don't see any value to adding the product if you are looking for "smoothness" or something intangible like that. I think really it has one purpose to extend engine life in circumstances where someone's actually going to see 250 three four 500,000 miles+ and any reduction in Iron wear or engine wear is worth the small expense of the additive.