1. It's a feature, not a product. The only use described, or that occurs to me, is screen recording. You could spend the time to make the best possible fit-windows-in-a-rectangle application, and the Camtasia folks could build it in to their application and make it fully integrated (align with their magic recording rectangle), and all your effort was for naught.
(In the Apple world, this has a name: "Sherlocking".)
2. Resizing a window is literally a single command -- at least on the Mac, and I assume also on other modern platforms (wmctrl, etc). I've got a bunch of these in the "Release" folder of my application for positioning windows for automated screenshots:
It's not as easy to use as "click a button" but I bet I could position 3 windows faster than you could buy and download an app online. I can queue up preset sizes/positions of windows before a screencast, too, and change them during the recording. Unless you specifically want to show off the maximization animation ("Welcome to 1995!"), you can just segue over it ("Now let's look at something else...") while you double-click that shell script off-screen.
(I say this not to suggest that everyone using a GUI app should just write shell scripts, but to point out how easy it would be for competitors. This would be the opposite of PG's "run upstairs" advice.)
I know what software should be written, but, like this idea, they're usually specific features of existing software that I don't have the source code for. What I have trouble figuring out is something to build which also has a viable revenue stream attached.