In with my tuppence worth. When I bought my E92 I didn't spec nav or bluetooth. Firstly being a tight wad, I felt that BMW were taking the piss wanting a shade over £2,500 for these items. The bluetooth didn't work with a great range of phones, and the nav didn't accept full post codes so they were inferior products at a superior price.
But at least it was mildly better value than the five series because you paid just about the same when you already had i-drive and a screen.
If it had cost a bit less I would have taken it because it isn't just sat nav but the entire centre control system for the car covering navigation, air conditioning/heating, sound, and telephone control.
With the bluetooth telematics selected also, you get the emergency services system (if an air bag goes off it phones the emergency services and gives your location.
It has a hard drive and a DVD player so your record collection can be downloaded to the car - no need of discs or multi changers in the boot. You can download internet stuff, updates for maps, you can send routes from your PC to the car if you want and I believe there are quite a few more functions which may or may not appeal to you.
So, you get quite a lot for your cash above and beyond the nav. It isn't really two grand for Nav alone, and I fancy the other facilities that are now available (since I bought my E92) more than I do the Nav which I have only needed once in the last five years.
Despite your valid comments about dealers maintaining that options are for your own enjoyment there are several options that make any car more easily sold. Leather, air conditioning, and built in sat-nav are three of them without question. (Indeed, if I was buying a 2nd hand E92 I wouldn't have bought one without it - maybe because there would have been no £2,500 extra label staring me in the face).
So I would see the positives in selecting it as all the other facilities that come with it, the integration of the system, makes it easier to sell when the time comes (private or trade) and you will actually get part of the extra cost back.
Negatives: if the facilities and integration don't appeal to you in the slightest, you rarely need the sat-nav, and you don't care about voice control and don't need hands free phoning, then paying thousands more for your car would indeed be madness.
There is a case for both sides of the argument and it's what suits you that counts, though I get the impression the salesman has bugged you by his attempted insistence on specifying nav - especially with such a large price tag!