Bad drivers will crash regardless of the reason. I don't see how this is any more dangerous than trying to drive while looking at a paper road map or a Google maps printout thereof. This will be great for all of the times that I get lost only to find I don't have a map in my car. And sure, I could simply buy a map for my car, but where would the fun in that be?
As for the traffic congestion, I don't see any danger in looking down at your cell phone and plotting out an alternative route while you're parked in gridlock rush hour traffic.