I shoot eagles fishing with the D300s and I highly value the fast frame rate. With each fishing pass I am trying to capture 3 or 4 specific images.
1. The moment just before the eagle gets to the fish.
2. The moment he grabs the fish.
3. The moment just after the fish comes out of the water.
4. If possible, the eagle looking down at the fish in his talons or possibly eating the fish in mid-air.
These 4 events usually happen in less than a single second.
That said I do not use the grip on my D300s. That one extra frame would have little impact on the shot sequence and cause the buffer to fill just a bit quicker. If it were a 2 or 3 FPS difference I think it would probably be worth it to use the grip.
Many people use the grip for the extra battery life and because they like the feel/weight of a bigger body.