Not specifically, no, but the power clean is an excellent exercise for attempting to train power. It cannot be done slowly and so requires the trainee to accelerate the load in order to complete the lift, unlike, say, a speed squat, where the lift can still be finished in any amount of time. The clean can also be done with a lot of weight, so the trainee is forced to use maximum power production to complete the lift. The snatch is similar, but lower weights are typically used (for me at least), though the range of motion is greater.
I don't know if it degrades, but it seems intuitive to me that in the twin case above, the twin who did train the power clean maintains his ability to produce pulling power, whereas the other would have to separately train the clean to achieve this. Maybe someone who never trains the clean but can deadlift 405 lbs can just automatically clean 225 lbs. Not sure.
Now, does it have to be maintained strictly from a strength training standpoint? Personally, I think so. I think it can be useful for helping the deadlift, since the deadlift is the only lift that begins on the floor, with the lifter completely unloaded. The squat and the bench use a stretch reflex which helps with "coming out of the hole", and the press has the little hip motion to get the lift going. The deadlift benefits from neither of these, so raw pulling power must come from sheer commitment to the pull, which I think the power clean would help develop. I'm not a very experienced lifter, but these are my thoughts.
I really have no idea, but much like strength development, the approach is to develop the modality in a general sense, and I guess assume it translates well outside of the weight room.