I can charge it while using nav but that is not the point. The point is that the battery is wearing down much faster than it did prior to MM.
Ok, but my point is that you have to manage the battery life that you have, so not plugging in while you are doing one of the most battery intensive things that your phone will do is a wasted opportunity to manage the best battery life. It's hard to plug in the phone while you are walking around, or at random times during the day. It's quite easy to plug in while you are driving your car - so do it. You will definitely arrive at work and have a battery charged more than 36% at noon.