It seems to me that BMW is positioning the z4 for that baby boomer crowd who has become successful in life and want a luxurious but not harsh sports car experience. You know - something that is more exotic than a Corvette, but less splashy and expensive than an Aston Martin (or you fill in the blank).
I would guess they plan on producing fewer of them too. But the Z2 will take the place of those who lusted after the more sporty buzz in your seats experience that came from the Z3; yet do it with more pizzaz, more luxury (options) and considerably less money than a well equipped Z4. It's product differentiation. We're all just used to one 2-seat BMW sports car (the Z8 notwithstanding) and many of us are chagrined over the high MSRP of this fabulous machine.
Unless BMW comes out with substantial manufacturer discounts, I doubt my next car will be the Z4. This might be the push that gets me into a C6 instead
Doug