I get nav because usually the nav comes as part of a package that includes a bunch of other equipment. On various cars I've seen over the last few years, sometimes the nav comes bundled with an upgraded sound system, rear view camera, upgraded interior tech package, etc.
From an aesthetics standpoint, I think most cars look better with a nav screen than without. That doesn't really apply to the A3 and other cars that have a retractable screen, but I think the ones with fixed screens look better with it. Even the ones that offer touch screens without nav generally offer a bigger screen with nav, and that almost always looks better in my opinion.
As far as all the other stuff you mentioned the iPhone having, I don't particularly want any of that in my car's nav system, besides music and nav. I have a phone that does everything else already, so why would I want to text or check out LinkedIn through my car?
All that said, I would love it if the automotive manufacturers went with a customer accessible, modular infotainment system and rolled out regular updates like the phone manufacturers do.