Not sure I can manage a metaphor, but it seems pretty obvious that RChung is right about the fundamentals, and that it makes no sense to tell a new rider to aim for a particular cadence. Cadence is a function of power and resistance; paddling along like a hamster in too low a gear isn't going to make you fast, it isn't even going to build much strength; it's just going to exhaust and discourage. By all means encourage new riders to avoid using too high a gear (hard work, bad for the knees etc) and encourage them to use the gears to maximise their speed for a given amount of effort, but their achieving an efficient (and probably faster) cadence will be a consequence, not a cause, of their doing these things and becoming stronger riders.