The devices are already ultra thin and super light...with the battery #'s we get now, i can't imagine I would personally want to sacrifice anymore battery longevity for a fraction smaller chassis that is really only noticeable when the two items are set right next to each other. I also know that eventually, technology will be there to make the batteries paper thing and the devices will, in turn, become significantly thinner and lighter...until that time, what we have I am content with and would be for at least a few more years.
Making a device too light also has a very direct effect on the "cheapness" FEEL to the device...the iPhone 5 (coming from the 4S) was shockingly light, almost to the point where i was uncomfortable carrying it around, for fear of breaking it or the quality of the build. The Macbook Air is the same way...it feels so light, i personally start questioning how durable it would be in my day to day activities.
One thing that I also want to be comfortable about with a device that is designed significantly thinner and lighter is it's durability...i dont want to have a device that gets dropped from a couch cushion and ends up bent or dented significantly, I also don't want the glass panel on the device to be fragile to where years of use causes stress cracks or shattering issues (after the device is out of warranty).
The retina display is there...i would not want to move away from that on the iOS devices, so that is a must have for me now. So in the end, if i had to pick one of those three things as "most important" as our devices sit right now, it would definitely be the retina display.