Mostly using Flash for the whole site makes it shitty to use and leads customers and users to other, more user friendly sites.

I think it's more about wanting to have the freedom of having both HTML5 and Flash work.

While certainly that kind of statistics would be interesting, what would detract people from gaming the system? Easy enough to start to skew your contribution to get more prominent place in the grand contributor list.

And that makes it pretty hard to improve FOSS UIs without company backing all the usability testing that needs to get done. Where as the coding part can be done reasonably cheaply, usability testing starts to cost as it's pretty hard trying to find anyone to participate for free. Essentially any usability expert would need to put in his own money in addition to his own time to contribute.