I did a little research and found this to be very interesting.
(It is a .PDF file)
Admittedly it's an older paper, but since it was written when 7200 RPM drives were emerging it's easier to look at that information backwards to see what a step back a slower drive would be.
It makes me want to sit tight and enjoy the better performance of my existing drive while the industry works out a larger capacity 7200 RPM drive that will fit within the Duo.
By the looks of things it shouldn't be long.
It's still an interesting idea, but it's also interesting to throw around the pros and cons IMO.