Monoid instances for mtl

Hi all, On a number of occasions I found myself declaring simple instances like instance Monoid a => Monoid (State s a) where mzero = return mzero mappend = liftM2 mappend that make my life considerably easier. Would it be a good idea to add such instances to the Monad Transformer Library? Of course, one could ask where to stop---but for me it make sense to always add sensible instances of the classes from base to a given library. Cheers, Stefan