I'm not doubting you've had success with it, in fact, its not out of the question that you would. However, I've had success with a Choice Band Kingdra before. The ladder is rather incompetent at the moment, and hasn't been stellar for a while now. All Giga Drain does is cut down on coverage. If Roserade somehow managed a move that could maybe combine the effects of Hidden Power and Extrasensory (without using a different Hidden Power of course), then Giga Drain would probably get the nod there. Keeping its HP up while ploughing through a team is pretty cool, something most sweepers don't have access to.