I'm not sure I want everybody to know where I am at every given hour of the day....
No, but being able to push a button and inform all of ham radio where you are sometimes as needed is very powerful. Like portable Sat operations for example.
Besides, the iPhone app would not currently be practical for many common uses of APRS. If you really want vehicle tracking, for instance...
Forgive me for stumping like this, but that is a common misconception. See www.aprs.org/APRS-tactical.html
APRS was never designed to be a vehicle tracking system. It is a fundamental digital communications backbone network between all operators in a local or global area. No matter what, where, or how, or on what frequency someone is operating, they always have a digital channel to any/everyone else in ham radio on this single national channel.
It's the signalling and info channel for everything. Oh, by the way, yes, it can be used to place you on the map. But in most cases no one really cares where you are... But the end-to-end connectivity for simple text messaging only by callsign is very powerful. See www.aprs.org/aprs-messaging.html
Its supposed to be a resource for seeing what are all the ham radio assets, systems, and nets, and operations are in your immediate area live, now.
Sorry for the stumping. Bob, WB4APR