> dont quite see the point in this.. and why chrome ? and why have to sign up ?
It's MAME, in a Chrome tab, any place Chrome runs (including potentially Chromebooks and Android devices in the future), and you don't even need to download a rom to try it out. If you don't intuitively get why that's cool, I can't help you.
Why Chrome? Because the technology necessary to do this only exists in Chrome. Adobe showed something similar a few years ago that would've worked in any browser with a Flash plugin but abandoned it before release.
Regarding the sign-up: it'll accept any Gmail or YouTube or other Google-affiliated account. I don't know many people who have Internet access and not at least 3 of those.