I use the admin bar on all of my personal sites, mostly to make it easier to jump from one to another. On one larger client network, though, I had to disable the admin bar on the front-end. They're using a theme that has its own version of the admin bar built in to the front-end. Really, it's a PITA because it was poorly executed and majorly borks the presentation of their home page for logged-in users.

But in the end, there can only be one ... and it's easier to disable the new one that ships with WP than it is to re-write a poorly-designed theme.