I also can't see why I'd want a built-in satnav, though I wouldn't mind having a cradle that connected my phone to a larger display. The one advantage to builtin satnav is that the display can be clearer and more informative.
I own an add-on Garmin, and while I like it, I use my phone far more often. Among other things, it has a much better understanding of traffic and how to route around it. (My particular Garmin model does get traffic information over the air, but that information is much less precise and useful than the Internet can give it.) I keep it around for those days when I'm out of coverage, and a few other circumstances, but it's mostly relegated to taking up space and getting out of date. (It also has map updates, but they don't happen automatically. And a major new interchange near my house took a very long time to appear. I didn't need it, but people coming to my place sometimes got confused.)
So I can see why a car my want I/O features, but the smarts might as well be relegated to the device I'm already carrying with me. It would be nice to have better audio controls than fiddling with the phone itself, for example.