Do you think it's the "market" or the "marketing" that has put the focus on being lighter and lighter each time?
I think a substantial portion of the market would trade some weight for much longer battery life, at least in the iPhone realm. OTOH, a low-power CPU can make all the difference--i.e., the new Haswell MacBook Airs! Same weight, but incredible 12-14 hour battery life--even better than the iPad's!