If you can eta reduce a type synonym, it is always better:

-- Bad
type F a = Maybe a
instance Functor F -- illegal, partially applied type synonym! 
-- Good
type F = Maybe
instance Functor F -- ok with type synonym instances

So... GHC should do this automatically for users. Ordinarily this is not terribly important (thus the priority); in the context of #12680 it is a modestly helpful affordance which allows a user to implement data Map a using type Map a = HashMap a (signature matching requires the type synonym to be nullary.)