I think that the imperative can be interesting, but the fact that it completely dismisses the consequences of an action is worrying. Take the following as an example: A man sets out with the intention of helping a friend out. The situation is that the friend has furniture and other such appliances that need to be moved into a new flat (upstairs apartment), and he needs some help.
This man, lets call him Steve, and this friend lets call him Bob. Just for simplicities sake.
So Steve arrives at the entrance to Bob's flat and calls out to Bob, Bob tells him, correctly, that Steve has just left the hospital and isn't in the right shape to help him out with the heavy couches and such. Steve insists with the best of intentions and that he can in fact help move the couches. Due to the insistence and the fact that Bob does need the furniture moved inside he abates.
During their trip up the stairs Steve drops his end of the couch and Bob is rammed against the railing, the entire couch falling onto him. Bob is badly hurt. While it is true that Steve was acting in a manner that he would expect other friends to act, and had the best of intentions, the end result was simply not worth the help Steve would have received had he not dropped the couch.
My point here is that there are all kinds of scenarios where the intentions will not justify the means. Also, a person's could simply think of their outcome of the situations, the best and be wrong, essentially that people will have good intentions, yet not align those good intentions towards what would justifiably be moral. Perhaps Marx had the best of intentions whenever he wrote his doctrine, to help the suffering worker class, and yet sparked famines, death, and war in Russia.