[Haskell-cafe] instance Monad (Except err)

Hello, Mr. McBride and mr. Paterson define in their Applicative paper:

data Except e a = OK a | Failed e instance Monoid e => Applicative (Except e) where ...

Sometimes I'd still like to use >>= on Excepts but this "feels" wrong somehow, because it doesn't use monoids nicely like the Applicative instance does. Are there any good reasons such a Monad instance shouldn't be defined? Does it violate any laws, for example? Thanks, Martijn.