Rear wiper.
 

I noticed U.S. spec cars not equiped with a rear wiper,actually i prefer that,so that cleaning the rear glass will be easier.....

That is true. I have yet to drive my Z in the rain so I am not sure how bad the water build-up is back there. Wonder if it's actually worth having.

I've driven in the rain a couple of times. It's really not an issue because you can see so little through the back window anyway. Most of the water seems to blow off unless you're going very slow.

a rear wiper is always nice to have. i'm guessing this was a way for nissan to save a few bucks...

I think it wouldn't add much. I've driven quite a bit in the rain. A good coat of RainX and it's a moot point.