Seefacts, I think that his face is considered perfect for those kind of characters (or at least what he can do with it).
Whether it actually IS or not is down to personal opinion.
And as good as Nicholas Lyndhurst is, yes, part of me can't help agreeing. I'm not sure if it's so much the voice as the face for me though. I guess he'll always to some extent be known as Rodney, no matter what he does.