I think it's a consequence of one of the points TFA brought up - WIMP systems were originally designed to approach modelessness, but users want modal interfaces. They want to go into the context of an application and then switch into a "mode" where they only need to know about the keyboard shortcuts, UI idioms, etc. of that one application.
Sometimes you can't accomplish a task like this, but on the frequent occasions when that happens I'm reminded of exactly how annoying it is to, for example, look at a screen that's split between browser and terminal and switch back and forth between the very different interaction paradigms the two require.
Sometimes you can't accomplish a task like this, but on the frequent occasions when that happens I'm reminded of exactly how annoying it is to, for example, look at a screen that's split between browser and terminal and switch back and forth between the very different interaction paradigms the two require.