Indeed the data type linked to is trivial and isomorphic to U1 as well as Const (). The more interesting question to ask is: Can monads be used to securely erase effects? That is, for what monads is
action1 >> action2
observationally equivalent to action2? That question comes up every now and then. Turns out that holds for a surprisingly small class of monads, the reader monad being the prime example. See also here.