Just out of curiosity (I'm new to the iPhone world with a 4S) why can't you use the device as a phone/entertainment system simultaneously? Movies/games I can understand as you'd have to pause to look at the map, but music will just keep playing in the background. The reason I ask is that Garmin puts out apps for $50-60 that download the maps to your phone (so you don't need to be in a cell coverage area to use it). You could save considerable coin by going that route but if you're dead set on a handheld unit it's your call.
There are a few reasons.
1. A dedicated GPS has a mount which means you can mount it on your dash and always have it visible out of the corner of your eye if you need to see it.
2. Your iPhone GPS is going to suck down its batteries so it would have to stay plugged in the whole time. (which probably isn't a big deal to most people)
3. For my personal taste I like having my iPhone to be used as a phone when I need it. I spend a lot of time on my phone talking on long road trips.
4. The GPS will just do a better job as a navigation device. Better routes, speaks out directions to you, more accurate instructions, more aware of where it actually is, better at announcing future actions, will easily show you how much time / miles are left on your trip, and they do handy things like tell you how close the next gas station is.