[Haskell-cafe] omitting params in function bindings

Is there a deep reason (beyond saving a sentence or two in the language definition) for requiring all patterns in a function binding to have the same explicit arity? For example, in dropWhile0 :: Num a => [a] -> [a] dropWhile0 (0:xs) = dropWhile0 xs dropWhile0 xs = xs why shouldn't the last line be replaceable by dropWhile0 = id