Cadence is important, but it's also something that people tend to figure out on their own. You can feel what your legs want to do.
The only common exception seems to be when cyclists convince themselves that bottoming out gearing on a climb isn't a performance issue. But that's also not a situation that a cadence meter will help with; the issue isn't that the person doesn't know that their cadence is low, it's that they don't acknowledge the power penalty. In other words, a power readout would be much more informative than a cadence readout.