The Mozilla foundation has our best interests at heart much more than Google and while it's not C++, html5/javascript is certainly an open enough platform which basically everything has started to target anyways.
Sure, a pure linux phone would be cool in some ways, but FirefoxOS seems much closer, realer, and every bit as awesome.
The purpose of reimplementation is generally to screw consumers. There's nothing special about a phone that requires you to put everything into a VM. I don't know why firefox is doing it. I did order a development phone, though, because if it takes off, I want a share:)
To make it more accessible. Way more people can make web page than make QT or GTK GUI. Way more people can do Javascript than can program in C or C++.
That means, that everyone is stuck with html and js, but I think it's worth it.
Linux phones are real. I've been using one exclusively for the past 5 years.