I think cadence depends on what you want out of cycling. If you just want to cruise around, it really doesn't matter all that much. If you want to ride longer distances, tour on a bicycle, ride fast, etc. cadence is more important. I would suggest however, that if you want to avoid knee injuries and your cadence is below 75 rpms, there is room for improvement. Improvement can come over time. I know many touring cyclist that rarely get into the 90s with their cadence, but feel less efficient when dropping below 75. I learned to up my cadence over an entire summer trying to increase the numbers no more than two or three rpms per week. If I was riding at 60 rpms I would attempt to ride for 10 to 12 minutes at 62 rpms. Then I'd increase it to 15 minutes on the next ride. The point is, you can increase cadence is small steps.