They've already been phased out of use in the North (they're still in existence but no one refers to them anymore). Maybe they serve some purpose in larger states where if you say "oh I live in such and such town" - and people have no idea where that is, giving them a county might help them figure it out.
At least in MA when describing a town someone doesn't know, we simply break it into regions (Western MA, Central MA, and then the eastern side is broken into North Shore, South Shore, and the Cape/Islands). Easier that way than to say whatever county it's in, and then have the person say "oh cool, that towns in Norfolk County.... where's Norfolk County?"