The problem with "on model" is often times it turns into "taken directly from the style guide". And I've always been an opponent of style guides, because they never look the exact same as the characters anyway, especially if it's a cartoon. Then the characters tend to change their looks subtly from the first through last episodes of something (though it's becoming less and less in a digital era). Plus, while there would be a lot of artists that would do it justice, a lot of them would just slavishly copy stuff, not giving enough room for movement or fluctuation.
Personally, I subscribe to the Kent Butterworth (I think that's the guy... this was based on a quote from Milton Knight's website) philosophy that if it's recognizable enough, it's acceptable.
We all have to admit, Gonzo IS pretty hard to get down. Not just the nose, but the eye placement, and the various little subtleties in his structure... Big Bird's the same for me... he has so much subtlety in the shape of his head alone. Plus, you gotta admit, Amy's Gonzo actually looks like the current puppet a bit.
EDIT:
Uhhhh... Is Sherlock Holmes gonna be the next thing? I just heard there's gonna be a Tom and Jerry meet Sherlock Holmes movie on DVD...