No system requires monad transformers or extensible effects. In some cases they become useful, particularly when dealing with unusual computational contexts; but most of the time you can use IO, and sacrifice the sharpest edge of type-safety for an easier job of plumbing.
Yes, but as I was saying logging, errors and so on can all be handled directly inside of IO. Given that you can't do any of these things in a pure function anyway, the only loss of dropping into the wider context of IO is type safety.
