Because I sometimes use my phone and its screen for well over 4 hours per day, using Waze, taking notes, sending messages, using email, researching, playing games, reading Reddit and tech blogs, and even talking on the phone. With my current phone (OPO), I can get over 6 hours SOT, or well over 4 hours SOT plus 6 hours tethering (the Note tablet I use at work, syncing to Evernote), 3 hours bluetooth streaming, and a lot of calls. This is either on SlimKat or on SlimSaber LP built with CAF. I have no doubt that the MPE will soon have some great roms that will have way better battery life, and google will finally solve the MRA bug, so the Moto X will also get 6 hours SOT. I will wait until then. The main reason I want the MPE is storage. I am way cramped with 64GB on the OPO. But battery life is even more important than storage to me. Having an unlockable bootloader is the most important of all, otherwise I would have a Note 4 to also get the replaceable battery.