I Know that Halo 2 on Games for Windows does not support splitscreen which is a big part of the halo series.
I Understand why it doesn't support splitscreen (one keyboard and mouse) but people like me can use xbox 360 controllers to play Games for Windows and multiple controllers could make splitscreen possible.
for people That dont own 360 controllers or their USB wireless adapters could always plug in multiple keyboards and mouses.
Though it most likely wont happen, halo 2 pc splitscreen would make the game alot more fun and would give the non LIVE players another reason to get the game.