It's the trend started by Android and then followed by iOS and also begun on OSX. It's not quite required but you're pretty much going to want a cloud account.
If I remember correctly, most of Modern UI apps won't be accessible without a Microsoft account.
Well you can't get into the store without a Live account, and Modern UI apps can't be installed from outside the store.
You can sideload them.
Apps that require Live services require a Live account I imagine, but nothing else seems to have.