This could be used in several things. I personally thought of the idea because of my love for silent hill. I customized all my sounds and music to fit the silent hill theme as well of the textures. This would revolutionize fantasy maps in a whole new way.
Actually... This might be possible in the upcoming patch. I thought I heard something about a command for loading a resource pack in 1.8.
It'd still probably cause a ton of lag, especially on older computers, when the switch was instigated. And if there is no way to swap out resource packs during gameplay, then you'd probably have to resort to large-scale block replacement. Which is another lag monster, not to mention would require essentially copying every block and item in the game. Or, at least, all the ones you want to change.
It's been possible, there was a mod that did it, although I can't remember the name of it right now. I'm uncertainly sure that it would have to use the onTick method in blocks. It would cause a lot less lag than creating a TileEntity, and using that due to the fact that tile entities get ticked, every single tick. Instead blocks get added to like a pool of blocks that need to get ticked.
Imagine trying to get out of a pool on that ladder thing, you have to go one at a time; same concept.