But if you're recording both channels, that situation is largely unimportant. You can just jump back and forth between programs all you want and skip the commercials.

Unless of course you want to actually view two things on the same screen at the same time as Tony mentions. Personally, I get bothered by watermarks and other on-screen graphics, so a PIP rect would probably drive me up the wall.

It's more convenient to be able to stream the second channel to a device like an iPad.