I ask because the police appear to be using this capability in actual investigations. They have a subject, they want to follow the subject. It's not a dragnet kind of thing.
The NYS Department of Labor and the NYS Office of the Inspector General placed GPS devices on the personal vehicles of an employee who was being investigated for timecard abuse. (http://www.aclu.org/technology-and-liberty/nyclu-files-suit-...)
This isn't exactly the same, but I think the issues are similar enough for discussion purposes. Police do have the ability to have investigators follow someone to learn about their behavior. But the costs associated with doing this, both in dollars and investigative resources, are high enough that police do not follow subjects 24x7 often -- certainly not to investigate some state employee's time card issues. How low does the bar go? Will the cops track me if I'm suspected of illegally obtaining fireworks? Jump a subway turnstile? Shoplift a candy bar?
This relatively new technology lowers the cost curve so much that the barrier to entry is trivally low. And that is a problem that must be moderated by some power external to the police.
Is anyone harmed? I don't know how you could argue that having the authorities systematically track someone's movements isn't harmful. Even people as irrational as crazy ex-boyfriends track the movements of their ex's to intimidate. How would you feel personally if you found a GPS tracker on your car? Or discovered that your phone was tapped? I would feel sick -- whether I was a criminal or not!
As for your last sentence, the police have needed warrants to tap your phone since the 1950s, and need a warrant to place a GPS tracker on your car, so I'm not sure what the point of those examples is either.