[Endoperez]

Caves aren't used in numbers comparable to forests, swamps or even seas. Your point would be true if, and only if, caves were either

1) common enough to be just another type of terrain, like mountains of farmlands.
2) large and with many connections to the neighbouring provinces, like seas, thus forming important waypoints

If cave provinces aren't common and don't give the controlling nation the strategic movement of underwater races, it doesn't matter that your army can't take control of it. If they are, the map is probably somewhat unbalanced in multiplayer. To my knowledge, the only map that uses caves to a larger extent is the World of Geometry, but in that one caves are part of the story.

In short, I think that a map can be made so that having darkvision is a major bonus, but also so that the cave provinces are just a small bonus for several races, some of which really appreciate the help. What kind of darkness effect is in them doesn't really matter.