one benefit i noticed with using DB in this manner was to feel out shoulder position. since i still feel a good deal of elbow pressure when doing a supinated german hang (maybe its pronated cause the palms are actually facing the floor in this case hah), my shoulders tend to move in bad ways to compensate. this is partially what led to me having shoulder troubles and having to do rehab. i will do the DB holds occasionally because i can focus on keeping my shoulders in proper positions and work on increasing my bicep flexibility.
also after i finished shoulder rehab and eventually came back to muscle ups, even a kipping muscle up was too difficult. hanging upside down from a bar and using DB to simulate a muscle up helped a lot mainly because i could do it extremely slowly. otherwise it was just too hard to simulate the tension of the transition. also i played around with narrow and wide muscle up motions.