This is a very good question. I was wondering about the same thing before but didn't really put so much thought until you asked.
For me, the rear visibility isn't that great to start with, so somehow the lack of rear wiper wouldn't make much of a difference. Plus, I think the wiper may ruin the looks.
I did feel the need to put a water-repellent coating on the rear window, actually in coating all external glass. This indeed helped a lot with rain, the sloping roofline helps with that.
As for dirt, there's a solution. It's called car wash.
For reference, I could still see out that nasty rear window.
[photo=medium]692[/photo]