I personally disagree.
I looked for a car without the Nav unit. I was after a sports car. If I did, others will too. I also narrowed my choice down to 2 at the dealers. One had the Nav, the other didn't. The interior of the non-Nav car was better as it had the piano black/extended champaign napper leather, the other having the all black and silver dash interior (which I myself feels a bit dull and gloomy!). They were around the same price also.
Nothing worse a few years down the line when that unit is about as old hat as a cassette player and you are stuck with it. It may be useful for the other functions that screen does, but the Nav will start to look dated very quickly. I'm fine with my Tom Tom unit, and I only use that around twice a year! Just my opinion.
By the way, just done a national search on Z4 3.0Si's and it isn't really making a difference weather there is Nav or not on price. One dealer is selling a 2008, no Nav with only 100 miles on the clock for £29,000 which is more than a 2008 Z4M with Nav at another dealers!!
3.0si: http://www.bmw.co.uk/bmwuk/auc/car_details/0,,1260_159980106__,00.html?currentCh=WBADU52050LF69425
Z4M: http://www.bmw.co.uk/bmwuk/auc/car_details/0,,1260_159980106__,00.html?currentCh=WBSDU92020LG19040