Having thought about this a bit, you don't really base documents on Normal at all, it's more that Normal is attached to them if you don't explicitly base them on something else (in the same way that Normal is temporarily attached to them if the base template can't be located). Normal is simply there because that's the way Word works - it needs a template and if you don't supply one it (rather grudgingly) uses its own but it doesn't do the full template thing.
Thanks for the question - it's made me think - something I don't always do
