Interesting thoughts. I really do feel like the 5S doesn't really feel any different with or without a case on, it is THAT lightweight either ways. There are two things I can take away from that, first of which is that the light weight pushes the one-handed usage to easier levels, compounded by the already efficient length of the whole screen and the bezels, and second of which, it makes replacing it in my jeans pocket easier.
I like the current weight, but if they are going to do a bigger phone device in the (near) future, I can't see myself taking to the lack of heft if that happens.
All in all, no issue with the current weight, but please do a slightly heavier device in the future if the screen goes bigger, perhaps closely resembling the S5 or the M8's.