While using UV mode, there are several limitations I'd like to overcome for very complex terrains (specifically think of cave systems in this discussion).
In all cases, the vertex color support should allow choosing arbitrary mapping channels as well as allowing just sub-channels (Red, Green or Blue).
Masking: I want to be able to exclude or include areas of surfaces based off of Vertex Colors. This should allow the artists to use vertex paint to clear paths. This control should probably go in the Surface rollout.
Diversity: Add checkbox next to Map Channel for "Vertex Color". When on, the color id map is derived from the vertex colors of the specified map channel.
In the case of diversity, the use of vertex colors would save many problems with using bitmaps. Not only that, but it would add a ton of utility in Max 2017.1+ with the DataChannel modifier.
I personally consider this set of features very high priority.