I think the most useful feature is to drive down the prices of the D3 and D700. I'm still not taking many pictures where I say to myself, "Gosh, if only this were a 24 megapixel camera."
I think you are right. I really hope Nikon stay with the present D3/D700 sensor and maintain this line of cameras concurrently with the 'x' line. Nikon would then arguably have the two best in class cameras with the 12MP based 'street and sports' camera and the 24MP 'studio' camera. As great a camera as the Canon 5DMKII appears to be with it's video feature, I think Canon have made a mistake in not further developing a sensor in the old 5D range. By going with a 21MP sensor in the MKII they have adopted a sensor that will never have the same potential (everything else being equal) in terms of noise, frame rate etc as a lower MP sensor. In terms of video, 21MP does not appear to offer any advantage over a 12MP, so if and when Nikon adds video to their higher end cameras, I would bet they do it with the D3/700 sensor, at which point there will be very little reason to choose the Canon, over the Nikon. Some of the images I have seen taken with the 5D MKII show no major improvement in noise performance over the 5D.
Working with my D700 topside these last few months, it is one of those cameras that I think is a 'classic', and I hope to get it underwater this week with the manatees.