I'm guessing the lack of multi-display "support" for games that only run on one display is due to how DirectX itself doesn't provide such support. After all, DirectX 9.0c was released in 2004, a decade ago, at which time multi-display setups are not even remotely close to being the mainstream equipment.

Personally I'm hoping Windows 10 will have proper multi-display handling for games but seeing how DirectX 11 is a 2010 technology, I'm already not betting my money on it. Proper seamless multi-display handling for games will probably only be possible in 2025 or later, when games are starting to default on 2015-2020 technology.

See how FFXIV is only now going to release the DX11 client, and you'll see how 2025 isn't really so far fetched.