Do you have examples of good images made this way available to the public? I've only ever seen it used sloppily with the intent of hiding bad composition using a fancy technical effect, so it'd be really nice to see something else!
I assume one flash at the start and one at the end of movement
How do you know their intent? Composition is largely a matter of cropping if you already have the basics down. On top of that, what is perceived as "good" is often very subjective, especially within art. Personally my main intent for using the effect was the vivid colours and the interesting streaks and shadows they make in the final image, and not really to cover anything up. The effect brings a certain life to the photo that a regular still just can't present, especially considering how straight flash photography too often comes off as really dull when you are forced to use only the hotshoe. But there is far more risk involved when using the effect, because when you move the camera, you often cannot be certain of the end result. Even more so with concert photos. They are made in an often very chaotic environment where you have little control, and where you more than likely are shoved around by the crowd as well. The slow flash really accentuates that, and you have to hope your movement syncs up with whatever is happening on the stage or around you while the shutter is open, which can be for quite a while depending on your settings. But then for that reason I think it's the most rock 'n' roll way of taking pictures. On the flipside the images does come out as very stylized, which no doubt isn't everyone's cup of tea.