One reason, already mentioned above is the 10 pin connector. If you do any macro work the cable or electronic release is a must (for me anyway). Can't do that without the MB-D100.
I don't even notice the "plasiticy" feel when it's on the camera. It is rather squarish around the finger side when holding in portrait, but it's not too bad. (it is not as comfy as the f5) This is minor.
I am trying to figure out how to disable the 20 sec limit on recording. Doing the kind of street photography I do -- this would be invaluable.
But I wouldn't worry about it. If you get to a point where you really need it (as I do with all my new equipment purchases ) you will get it. It's called the nikonhavetohaveitus disease. Rarely fatal, always costly.
Oh, yes, the extra battery deal is a very nice feature. Plus you can even switch to emergency back up AA's if you need to.