I've seen these before, and never really though much about them. When I saw one this weekend for $10 at a game shop, I thought what the heck, it's cool to have even if I don't use it. It wasn't until I got it home that it occurred to me that this might actually be very useful. Turn out that it's perfect for the task of what I said in the title: it works great as a pass-through for 2600 games that won't fit in an unmodified 7800!
For a while, I've been using a 7800 exclusively for 2600 games (to save much needed space and inputs) but now I can finally play those games that just wouldn't fit, like Jawbreaker!
Not sure if there will be any picture degradation yet, and whether the switch part is much use, but still it's pretty cool to me!