I've stayed at JW properties that offer different brands of amenities. I've also stayed at Marriott brands that have offered different brands of amenities. I've stayed at hotels where the suites got a higher amenity brand than regular rooms.
From a cost perspective, presumably a hotel company (any hotel company; not specific to Marriott) would get larger discounts on high volume purchases of amenities, so for some of the mainstream brands offering a single brand amenity might make sense.
If you prefer the amenities at FS or MO's, you obviously have the option of staying at them vs. JW or RC.
One amenity brand I'd like to see go away is Thann. Can't stand the stuff. I do like L'Occitane and I love Penhaligon's Quercus, which have been at properties I stay at.
Cheers.