I'm not making any points about users of mainstream languages. They have no use for transformers. Using a lot of global IORefs is possible but relies on a trick using unsafePerformIO; its definitely not in the spirit of Haskell to work this way though it is sometimes needed.

The main problems are immutability and inability perform I/O outside the IO monad. To address immutability you have Monads like Reader, Writer, State which can give you a similar experience that you would have with mutable data and/or global variables. For example with the state monad you can have code like:

  runState do
    value <- get
    put counter+1

The problem comes when you also want to do IO with that state; if your code is in the IO monad it has no access to the State monad and can't call those methods. There are ways to work around that but monad transformers give you a StateT which can be applied to any base monad, including IO, giving you the capability of both.

None. Transformers are a solution to the composability of monads.