It doesn't change the fact that you would be wasting development resources, because you then have duplicate functionality. Why waste resources on developing an addon, which will need to be updated with the browser api's, for many browsers, and optimised, when the browsers themselves already have the functionality built in? If resources are spent optimising the browsers themselves rather than a foreign addon, every website benefits.
Fact is, the same problem's we've had with flash addon in the past (such as in cases where updating the browser caused crashes), will still exist in Gnash player instead.
Why are you so eager to encourage people to duplicate functionality browsers are building into their core code these days?
You can either improve the browser, or you can improve Gnash. Both are open source. Unless there is something the flash standard can do which open standards couldn't?