I agree that spinning is supposed to be the "right" thing to do... however my partner, who rides in the top grade in the local club, and who came 5th in NZ in his class in the National champs last year, finds that spinning hurts his knees - and has, after many attempts, resigned himself to limiting spinning.

When I did some fitness tests recently - all hooked up to machines etc - they found that when I spin, my heart goes up, when I drop my cadence, and grind more, my heart rate drops. So I am more efficient at a slower cadence...