Short answer to the original post - I can't think of a single compelling reason to put Windows 7 on a cab.
I've used Vista since Beta 2, and am now mucking about with Windows 7. I do intend to do a little benchmarking, but haven't had the time to do so yet. My totally unscientific observations thus far - Windows 7 definitely seems to do better overall with how it handles resources, which is to say it isn't a hog. It boots a bit faster than Vista for sure. I like the tweaks they've made to the User Interface, though none of them are particularly groundbreaking. If you're going to bother messing with it, at least dive in and use the 64bit version so you can enjoy 64bit MAME (Which rocks by the way). I wouldn't put it on a cab just yet, seeing as how there have already been some nasty bugs discovered (it was damaging MP3files when played, but a patch is available), but then you wouldn't put data you didn't have a backup of on a beta OS now, would you?

As for why MS is so quick to be putting Win7 out the door ASAP? Corporate users have generally been loathe to adopt Vista, which has definitely not been good for the bottom line.
And as for Vista being so expensive? I paid under $140 each for both copies of Vista Ultimate I've purchased. OEM copies can be had easily, and are much less expensive than retail boxes. That's in the neighborhood of What Apple charges for OS X. Which reminds me, Windows 7 runs pretty nicely virtualized on my Macbook Pro at work too.