I see your point, but have a couple of arguments:
1. While I can see using the phone as a GPS, I don't like having the phone detached from my body. In the event of a wreck, I'm not going to want to have to drag myself over to wherever my phone is, if it's even in one piece.
2. Many tank bags have waterproof map compartments on the top which would suffice.
3. While I haven't done much research on the 5, the 4 has many waterproof options from otterbox and a few others, I believe.