Amin --I think you will see more than just enthusiasts question the need for a D3. I know from my own experience that spending a full day shooting an assignment with one of the 24-70 or 70-200 zooms leaves you fatigued. In any case, if I planned on shooting with the grip, I would just keep the D3, but I know from using the D3 and from using DMR that I hardly ever use the secondary release. It takes too much time to change the grip back and forth...I just stay with the one. The battery life is a valid concern, but how long does the other battery last? The battery life of the D3 is absurd to the point of being great to have, but not necessarily useful. I have shot over 700 images over the course of a full day, and the battery was still 3/4 full. If the other battery has half the capacity, it would still be plenty. And of course, an extra battery is light and easy to bring along.
As for construction, have you used an F6? It is built like a brick. I have a feeling that the D700 will be similarly bullet-proof. Jim Reed's account seems to indicate this as well.
I guess my point is that it seems that the D700 is every bit the pro camera that the D3 is, just aimed at a different market segment. Instead of sports or wedding shooters who need high frame rates and huge battery and card capacity, it is aimed at those who need a more compact body, but can take the time to change a battery or card if necessary. It is more a camera for travel photographers, photo journalism (not war photography, but more normally paced pj work), stock and general photographic work.