The commuter has a bell for the section of my commute that's on a MUP. I tried the voice thing, and too many people couldn't figure out what "passing on your left" meant

The commuter's bell occasionally gets moved over to a roadie for large organized rides and tours. I can only say "on your left" so many times, and I prefer to give some kind of notice before passing other riders.

Each of the MTBs has one too. A bell is a nice thing to have going into a blind corner, or coming upon some hikers.