Well, if my *only* constraint on a type is that it be an instance of Monad, then *all* of the code written in do-blocks will be applicable to a generic Monad instance.  That's the whole point.  :-)

Furthermore, you make it sound like this generic case scenario is incredibly rare, but in fact it is very common:  it occurs whenever someone writes a monadic transformer, which happens all the time.  Imagine what writing monadic transformers would be like if you couldn't always trust that, say, (>>=) was a well-defined operation?

Cheers,

Greg