It seems to be a NES emulator. So, I actually meant "ROMs currently being played." Similar lack of concern about people hacking their own clients. And it is in JS, so it's totally modifiable already, but probably with minimized JS.
I'm big on the "web applications should die in a fire." line of thinking. I've never been keen on running arbitrary code on my computer, even if it is sandboxed (build a perfect sandbox, and then, maybe). Also, it's pretty high cost in terms of overhead, downloading, ability to parse via computer, etc. Maybe if they didn't try to turn a bunch of web content into web apps, I wouldn't hate all web apps reflexively.
There's also the SaaS, forced upgrades, inability to own your own software problem I have with it, so it's possible that even if used well, I wouldn't like them.