Skip the cutscenes. If you don't want to watch the story, then don't.
The game itself in between is truly brilliant, and still very very long.
"The 360's DVD format plain doesn't have room for the 4 hours of pre-rendered HD video in MGS4."
Actually, I imagine a relativley small amount of the 50GB actually goes towards the cutscenes, as the majority of them (by length) are done with the game engine. Most of the 50GB probably goes towards the fact there's loads of huge, varied levels - a lot more than you get in any other game I could think of.
If you play through this whole game, you'll quickly see that the large file size is a LOT more than just videos.