The salt/pepper shaker was just an example born of imagination for what can be done with such a small device. By the same token, one could reasonably ask - what legitimate purpose is to be served by making transponders so miniscule to begin with? The expenditure of resources necessary to R&D such small devices is not something that your average corp undertakes purely out of good will. There is usually the promise of a big payoff somewhere down the road. What does one suppose the big payoff is / will be for having produced micrscopic-sized RFID devices?
Speculative imagination seems fitting for this particular topic; one that is merely one among countless many topics that we the masses are largely resigned speculate upon due to the lack of valid and truthful information otherwise being readily made known to the public.
As far as proximity of reader/receiver, I don't see this as an obstacle given the increasing ubiquity of average people littering their existance with electro widgets easily capable of housing hidden or dual-purpose operation.....from the TV and cable / dish boxes and cellphones and the whole littany of techno gadgets that litter people's lives, living spaces, cars and pretty much most other spaces private and public....... just like the incessantly blabbering surveilance box from 1984. Every where one went, they were nearly always within the view/eye of a Big Brother box.