The main benefit of the Nano flash is that it increases the bit rate, which is a main player in video quality. DSLRs don't have a high enough bit rate, one of the reason's why they aren't up to the task. The EX1-3 barely makes the bit rate grade at 35mbs, the minimum for broadcast usually. I agree that the Nano Flash is really for features and controlled documentaries.
I have used both the Letus and RedRock adaptors. They are massive and make filming a cumbersome task, not usable for wildlife and most nature work. They are popular because of the lack of depth of field, honestly, if I had to choose, I would pick a DSLR with video over a Depth of Field adaptor. Though if we had a big budget, we would rent a Red One.