<quote who="Piotr Konieczny" date="Mon, Oct 29, 2012 at 04:19:03PM -0400">
Anonymous or low activity editors can contribute high quality content, certainly, but quantity (and by extrapolation, most quality) comes from registered ones.
(Case in point: no GA or FA can be written by an anon, or a SPE; and most of the primary contributors to those articles likely have many high quality edits to a large number of other articles).
I'm not sure I disagree with your point but I think your evidence is unfair.
Getting an article to a GA and FA is more a measure of how well somebody knows the Wikipedia rules and system and is able to jump through them. Only very active editors will even know that there is such a thing as a GA or a FA. That, alone, doesn't mean that most good encyclopedic content comes from people that do. Only that the cleanup necessary to satisfies Wikipedia's own internal guidelines does.
Regards, Mako