Originally Posted by Retro Grouch
Think of it as a tachometer to help you know when to shift. Like driving a stick shift car you can certainly ride a bicycle without one. Your performance may improve marginally if you use it this way even for commuting.
Right to the point. Also like a tach, for most non-performance riders it is marginal. But only most. IF you find that someof the time you are shifting too late then a cadence might be worthwhile for you. But even in this case you might want to just try paying extra attention for a while and see if you develop better habits before you spend the extra money.