For me it's two reasons....i agree with this, but i think i'm in the minority since majority of the time, my phone remains in my pocket.
iphone automatically connects to the car once i get in and it just plays music that's in my itunes.
navigation..i just use the stock one that came with the car if i need it.
1) I use Waze every time I drive. I cannot see the screen to know if a cop is sitting and ready to get people if it's on Tesla's charger. I need it at a level that I can quickly glance at while driving. Looking down is just dangerous. Depending on what route I want to take, I'll try to always use the Tesla navigation, but sometimes it doesn't give me the options that Waze does. Waze is my 2nd choice for navigation in my Tesla. It's mainly to save me from tickets. LOL
2) I have a very hard time getting my iPhone to hit that sweet spot on Tesla's charger. It's the most frustrating thing and it's a known issue. I'll have to see if it's different when I get the new iPhone. Until then, I need reliable charging while I drive.