Former masher here, now an ardent spinner.

"Naturally selected" cadence is bullocks. The theory goes that since bigger rides are "naturally" mashers it means that this is what is optimal for them. Bigger rides are naturally mashers due to the fact that they have poor cardiovascular fitness and cannot maintain a high RPM. Just because its natural doesn't mean its efficient. You can get away with mashing on the flats but once you want start climbing spinning is really your only hope of reaching to the top. I learned to spin by getting a cadence sensor and focusing on holding a certain RPM until it became "natural" for me to spin, even without looking at my garmin. Of course its normal for your cadence to drop a bit when climbing but if you're in the 60's range you're losing power. In a rather simplified context, wattage = force x cadence. If you sacrifice a newton meter or two to gain an extra 20 RPMs (dropping down a gear or two) you're putting out a lot more wattage than grinding a bigger gear would generate.

Mashing a harder gear does have its benefits. A trick I often use is to switch to a harder gear on a climb when I am out of breath -- it shifts part of the work load from my cardiovascular system to my neuromuscular system without too much loss in forward momentum. Of course this is only a brief reprieve and not a sustainable way to ride.