Does Marriott really need extra brands?
(I've got the same demur with Hilton and Accor by the way)

Not to say it may be good to keep the brand names for a while in a transition phase.
However, in the long term there must be a significant difference to keep investing in different brands and each brand must have some kind of value proposition to a specific market segment. I've got the idea they have covered each and every market segment pretty well with the current brands (except may be B&B but I doubt this is something they want to cover).
And already know borders between the brand are pretty vague and some properties are outliers in their specific brand.