Light painting is one of the rights of passage photographers have to try at some juncture. I enjoyed playing around with it in the early days, but what surprised me is I have used the techniques commercially on several occasions; from creating better backgrounds to my portraits in dark locations to capturing English Heritage sites. The importance of knowing how to light paint isn't necessary per se, but it does help you understand how light works and how your camera exposes a scene.This video is a great demonstration on how one might go about light painting in a commercial setting. From my own experience I would say that a continuous light source inside a softbox is by far the best way to go about it; particularly if you're working with shiny objects like cars. Generally speaking, the bigger and the softer the light source the better, although there are exceptions to that rule such as painting in minor details. In this tutorial they tread a fine line that I identified while light painting a small and complex room last year; the line between too many and too few exposures.
If you spend huge amounts of time carefully light painting every inch of the subject meticulously you're met with two problems: the first is that you've got a mammoth composite job waiting for you when you get in. The second is that you can get a strong "artificial" feel to the final image; something along the lines of HDR. That's not to say that with the right retouching it can be avoided and perhaps even having more exposures puts you in a stronger position, but I would always err on the side of caution when it comes how many images you use.
Regardless, this tutorial is very interesting to watch and undoubtedly helpful to those of you wanting to try your hand at these sort of techniques. I would also echo the comment in the video regarding compositing: Mike Kelley's approach of layers masked out and set to "lighten" is very effective.