As the weight of the lens used increases, so does the pressure exerted on your right hand to hold the camera steady. I have been doing a bit of walking around for the last few days with the Nikon D90 and Nikon 18-200mm combination (without the MB-D80), and I must admit the camera is somewhat unbalanced.
Moreover, vertical shooting becomes uneasy as I found my pinky figures left in the air many a times. As the weight of the lens lean the system forward, your wrist is actually attempting to counteract that weight with an equal backward force. That naturally moves your whole hand lower the grip shaft, resulting in a rather awkward hand grip posture.
Now, the MB-D80 enhances the need to shoot with a heavier lens and significantly improves the grip posture for orientation changes. It's in fact, in my opinion, the first accessory you should invest in when you get your hand on a Nikon D90. Well, I did.