at the risk of sounding redundant, +1 to what Rod said again.
I shot close to 4000 shots in Tanzania with this technique on a 10-day Safari. When there was a photo op and battery died, delay was a second w/a power cycle to switch to the body battery, then shooting again. Once the photo op was over (or enough taken) and the Land Rover was off to find more animals roaming or laying around, I'd switch-out the dead grip battery. Then camera off an on again to go back to using the grip. I was out on Safari for 10 days. I'd often go through 3 - 4 batteries in a day. I think I took off my grip to change the in-body battery once in the 10 days, back in the tent when there's no pressure. That's convenience.
p.s. 600 shots per batter?! lol, more like under 300, sometimes near 200. But then again, I'd do in-camera viewing and editing to weed out the real loser shots while we were driving so that takes power.