I think a lot of it depends on what you mean by "understanding Monads". If "understanding Monads" means knowing where they fit in category theory (not simply w

I think a lot of it depends on what you mean by "understanding Monads". If "understanding Monads" means knowing where they fit in category theory (not simply where they fit in the Haskell typeclass hierarchy) that is absolutely not necessary to be productive in Haskell - I've worked with effective, professional Haskell programmers who have no idea what adjunction is. Understanding how things fit together behind the interface well enough to implement it isn't a high bar and (as you say) has a pretty high payoff as you work on slightly larger projects.