You don’t even really need to understand monad transformers in general to use just the ‘ReaderT IO’ pattern, and if you want to go crazier, free monads are both easier to understand, and more flexible.
I'd say using transformers are more the barrier between beginner and intermediate. Its not impossible to do serious work without them but I'm not sure its worth doing.
I'd agree in general and almost made this same point but it depends on what you are doing. If you write a transformer that is essentially just a specific kind of Reader (like ResourceT), then you can just follow existing examples and not really have to understand how to write all the instances yourself.
