Similar to all the other users here, I would like to disable split screen in Metro UI (though I'm not sure you're quite understanding peoples issues with it, judging by your responses so far...).
For example, I like using the tiles in Metro UI to open my internet shortcuts, using Tile Genie (since MSFT still hasn't created a way to customise tiles to launch custom actions).
While in Windows 8 the internet shortcut would open full screen in Metro UI, now in Windows 8.1 it opens with Tile Genie still in the left hand side of the screen. I would like the option to turn OFF this NEW automatic functionality in Windows 8.1.
I'm pretty sure you'll say it's not possible, as I've looked for a solution already, so please just ask the developers to consider these options for future updates. As much as split screen is useful for some user activities, it is unnecessary and obstructive for some others, as I've just explained above.
If you're still not clear what the issue is, or why, please say and I'll send some screenshots and pictures to illustrate further.