I think the "brand presence" thing is badly worded, but an entirely valid concern. You just need to compare the Ubuntu 10.10 install to stock Gnome 2.30 (I think I have those versions right...), and the differences are stark. It's not just background, and colour, but key UI fixtures are gone/changed/moved, the various panel bits work very differently in some cases, etc.
While they're perfectly entitled to do that, it's not Gnome by any meaning of the word that would be understood to an end-user. You couldn't write a book about Gnome 2 for such users and have it be *useful* for them without the literary equivalent of #ifdef's against large swathes of it.
I think that's an appalling shame, and it's pretty obviously contrary to what Gnome is trying to do - which is to deliver something that would appeal to end-users.
I'm not against the various extensions, and I think maybe Gnome worry too much - look at Firefox as an excellent example (which also has many of the same weaknesses wrt. API/version control). It works great, and most people only need a small hit of crack. It's just that different people need a different hit. I think it would be a sad day if any significant extensions started shipping by default, though.