Tom, those mtb mini fender things save your back side from getting wet from tire spray. While you will still get some splatter up the back of your shirt, 90% of it will be deflected and you will stay drier, longer. Same for the front tire, keeps splatter from hitting you in the face from the tire.
On the Mohonk gravel trails, I would think the mini fenders wouldn't be as resourceful as riding on pavement in the city.