Here's my take. As an immediate preformance enhancment you are going to be disapointed but it's not entirely useless for a commuter if you are looking to build more speed or range, or improve your fitness. The same would go for a HRM.

For your day to day riding gear selection/cadence is probably best done by paying attention to yourself not guages. I generally find that it is time to shift to a higher gear when I'm spinning fast enough that I start to breath too hard. Generally I will use as low a gear I can without winding myself. I know for sure that I am in too high of a gear if I can feel it in my legs.

Where these points are comes with a little experience, a HRM can help you get a feel for them. I found that there is a maximum heart rate which I can maintain for a long time and by paying attention I can take myself up to that point and stay close to it even without the HRM. This translates into maximum preformance/quickest times between two points and the most range. A HRM can also help you get a feel for how easy you need to take it occasionally for best recovery.

Cadence will tell you how fast your legs are moving. You can read up and find out what recommended cadences are but you will probably not be able to maintain the most efficient rates unless you have trained yourself to do it. The main benifits you will be get from a guage is help in maintaining a consistent cadence and an easy way to know your average cadence which which you may want to raise over time.

Recording HR, cadence, speed distance, etc. periodically gives you a measure of how you're doing.