All bikes for use on the road must be supplied with a bell. That's a law. Whether you keep it on the bike after that is your choice but that audible warning device might help with avoiding collisions.
If you're riding carefully enough you should be able to avoid most situations where you need a bell. I need to shout warnings far less often than I used to, because I slowed down a bit.
Good argument from the defence. Peds don't hear bikes coming, and thus cause hazards. Still worth fitting a front brake.