Quote Originally Posted by neuroanatomist View Post
Blinks. Basically, the time between the pre-flash for eTTL and a first-curtain flash isn't significant, but with second-curtain flash, the delay from pre-flash is enough that the reflexive blink of a subject will be illuminated by the flash, rather than being part of the ambient. Put another way, if you shoot people looking at the camera with second-curtain sync, you'll get a much higher percentage of shots with people caught with their eyes closed in mid-blink.

There might be other reasons, too, but that's the one that comes to mind...
Nailed it on the head. The issue is eTTL and blinking. With static subjects there is no advantage between the two. Also, at faster shutter speeds, there isn't much advantage to the rear curtain sync.

I have also read that with Nikon shooting in AV (or whatever Nikonian is for that) you will actually get a slower shutter speed than you would get in the exact same shooting conditions with first curtain sync. I'm not in a position to test that on my 7D, but Nikon presumably does that because the blur effect is what they're assuming you have in mind using rear curtain.