The faster frame rate was a big one and the fact that I could use all one type battery was another.
I did keep an enel3 in the body as a "backup" but I never ran the enel4 down enough for the camera to switch over to the battery in the body.
I've read that it is possible to use the battery grip with no battery in the body, but I never tried that. It could save you a little weight, but that's really negligible.