I'm not worried about the battery life aspect, but I absolutely hate it when devices decide that they're smarter than their users. This whole driving sensors thing screams of this.
Take in-car navigation systems for example. The designers decided "hey the driver probably shouldn't be typing things in when they're driving," so they disable parts of the interface, but they never take into account that there may be someone who isn't driving that's using the nav system. Now to work around the poorly thought out restricts, I have to stop driving so that the passenger can update the nav route.
If the Moto X phone decides that it's going to dumb down the UI when it's moving at speed, well what are the passengers going to do when all of their Moto Xs enter car mode?