A nice rule of  as to when you must use Monad and when you can use Applicative style:

How do you use Control.Applicative to write cleaner Haskell?

Update:   May be another way to look at this is to think of >>= as combinator that applies the mapping function then  flattens the result of application of  (a -> mb), and (in some implementations) change the computation based on the value of the "a".   In computations where the >>= is only flattening the types, it can be converted to an Applicative computation.