I agree, I don't really understand it either. If you need that many pixels on a PC, hook it up to an external monitor. At least you have the choice with PC's, not so with flagship phones. Do we know for sure say the G3 would have better battery life with a 1080p screen though? Phone (and laptop) batteries have gotten better despite the increase in pixels.
I will say I have NO idea why we need more then 1080p on a 5" phone though. I... can't think of any reason for it.