When I use second shutter
I find that second curtain is good for low ambient light portaits with long exposures. People tend to shift and move after a flash goes off. If the shutter is still open they create a large amount of blur around themselves. The comments about getting more ambient light with second curtain seem to be part of the story. The part that I think is missing is that when using longer exposure times it is usually better for portraits of close subjects to expose for the amount of background exposure you want and then at the last instant, fire the flash to capture the closer subject. It just seems that people being photographed hold still until the flash fires. With the flash on second curtain, people tend to stay still during the ambient exposure, and then move after the exposure.
Have done some portait work in extreme low light. I've consistently found this approach to be very effective. I also tend to use manual flash control under these conditions to avoid a preflash as people sometimes will start to move thinking that the photo is done already.
Hope this helps,
Jeff