You know what's great about have maps on my phone? I can get out of my car and walk and still have my route mapped in. Don't even get me started about the amount of features in Google Maps vs. in-car nav.
That being said; When I'm in the car driving, using the navigation package is much more seamless than doing it on the phone. My Mercedes is all voice controlled and works very well. Press button on steering wheel -> say "Enter destination" -> say "123 main st city ville" -> Route mapped. There's a few more steps to do this on mobile, so I prefer to use the car's navigation system.
A proper solution would be let me route voice control to my phone somehow; turn the car interface into a peripheral for my phone.