As for portrait mode...how hard is it to be supporting the camera with your left hand with your right hand on the grip? That and if you're using a tripod you could use a L-bracket and a quick release on the ball head. You could quickly change from landscape and portrait.
I always said the same thing....until I used a grip on a camera for the first time. It's much more comfortable for extended periods of shooting and much easier to adjust controls with the grip. It's something you don't think about until you actually use it. No, it's not difficult or impossible to use the camera in portrait without the grip, but if you do alot of shooting this way, it's worth the money. I'm glad it's an option on the D300...you can use it when you want (or need to), and take it off when you want to go lighter.
I don't understand folks who want a grip for their d40's or D60's. The main advantage of these cam's is their small size and portability. Throwing a big grip on them, kinda defeats that purpose. I think there is a small group of photographers that like to play the "my camera is bigger than your camera" game.