The one that comes to mind immediately is when you create a draggable element with text it's usually desirable for the user that click-drag moves the element rather than selects the text depending which part you click.
Removing the attribute would probably make things worse, as site operators then overlay transparent elements - making everything even worse than when it was just styled as such.
I'm honestly at a loss with unselectable text, but for example capturing the right mouse button is very useful for applications.
Anyway, yes, it should be easy to turn those things off site-wide, like it's easy to zoom.