I am a reformed gear-masher. I realize this is in the clyde section, and muscular power here probably dominates over cardio fitness in terms of natural capability, so mashing higher gears may seem more natural. I am not myself a clyde, but I am educated on the topic of exercise physiology, and thought I would contribute.
Spinning a higher cadence can be invaluable. From a performance standpoint, you can maintain an heart-stressing activity longer than one which primarily stresses the metabolic pathways of the skeletal muscle itself. Eventually, the muscle gets tired, and you will reach a point at which it produces lactate faster than it can clear it (lactate itself is a fuel, folks, but has undesired "side effects"). This is your lactate threshold (I like the older term of "onset of blood lactate accumulation" or OBLA, which really describes what is happening). This causes local fatigue, and stresses the systems by which blood acid is buffered. You breathe harder (usually a respiratory threshold corresponds roughly to OBLA) to blow off CO2, which forms carbonic acid in blood.
If you can maintain a constant, aerobic effort below or just above OBLA (so that lactate accumulates more slowly), the muscle experiences much less fatigue and the CV system is less stressed. At a certain point (high % of vo2 max), you breathe harder, but the oxygen you bring in is largely spent on accessory muscles (sternocleidomastoid, scalenes) helping expand the ribcage further. This effect is more pronounced for those with more massive upper bodies.
So...you can spin faster, using your heart and lungs and accumulating lactate more slowly. Or, you can mash a higher gear and rely on the powerful, but relatively short-lived, capacities of the muscle tissue. If you're only doing shorter rides, this works just fine. For longer efforts, spinning becomes more necessary. It should also be noted that your heart and lungs recover much more quickly than muscles do from near-maximal efforts. Blood lactate returns to baseline within a couple of hours after even hard bouts, while muscular soreness and fatigue can linger for days and compromise training.
Also, there is a lot of thought in the direction of mashing higher gears being harder on the joints. Not everyone will experience problems here, but these thoughts are out there. And they make a lot of sense given the kinds of loads one can place on their knees by the sheer force of the quads. Ever heard that knee extensions can be hard on the knee, especially if done over too wide of a range of motion? The loads that can be handled by the quads are why. Compound these forces (not identical I know, but similar idea) with the repetitive motion of cycling, and it makes sense that it could cause problems.
Again, I am not a clyde, and as I have ridden more and more my tendency has become to spin a higher cadence. This may not come naturally to everyone else, I realize. A person's neurology and muscle fiber composition may not suit this strategy. However, I think most people ought to at least try spinning a higher cadence.