Well I guess the meat of the problem is I don't want it being a phone and a GPS at the same time. I don't want a call to come in and interrupt my navigation at an inopportune time. Plus, I already had a standalone GPS anyway.
I have an iPhone and a dedicated GPS unit because I don't want my phone triple-tasking as a phone, music player, and GPS unit while I'm driving. I don't trust it to do that many things at once without them tripping over each other at some point.