I know Ashley has mentioned that he doesn't quite see the point in reinventing the wheel when it comes to certain tools from Tiled, but there is one rather big reason why having terrain mapping might be a good idea, and I realized it when the newest update included the ability to modify tilemaps at runtime.
I mean, with tile maps, inevitably, you're gonna be seeing people using it in level editors and games where you can modify the terrain in some fashion. Aside from making life a lot easier for those people if they want tiles to 'blend', it would also open up interesting possibilities for naturalistic tile-based terrain that can also be modified in-game, and I imagine there could be settings to identify which 'terrain' a tile properly belongs to, for the sake of games like Terraria, where you break blocks to gather resources, and stuff like that. And you can probably have terrain tiles that exist in cases where they don't 'blend' with the surrounding tiles.
tl;dr: Terrain mapping has plenty of in-game potential as well as being useful for editing. Food for thought.