do is only really good if you use it for the whole function, or at least have some way to get the error out of the do block to the code that's supposed to handle it. But if break isn't allowed inside a do block, then if you need to break you have to split up your do blocks into blocks before the break and after the break. Now if there's an error thrown by one of the statements in one of those do blocks, then you have no straightforward way to propagate it out of the function. You would need to pattern match on the result of each do block and return the error if there was one--in other words, you would need to write try!

This is why I called try! monads for imperative languages: the early return that is expands to is the key to playing nice with imperative constructs like break and continue.