I think are benefits to be gained by working on climbing (beyond just improved overall fitness). The steeper the climbs, I think the more it matters. I don’t think it’s just a cadence/gearing thing.
I hear some people say “watts are watts”, but there is definitely a difference in biomechanics when climbing an 8% grade vs riding on the flats at the same power and cadence. Some call it momentum or inertia and you often hear it discussed in the context of a trainer having a big heavy flywheel (more like riding the flats) vs. a smaller flywheel (more like climbing).
Riding in the flats, I always feel like I’m “on top of the gear” and my cadence feels smooth and efficient even spinning well over 90 rpm. Contrast that with a 10% climb and my cadence feels frantic at 90 rpm and much harder for me to maintain power for an extended period. I feel like on steep hills, you really need to apply force more constantly throughout the pedal stroke because you have the constant resistance of gravity to fight with. On the flats, I’m guessing that there are bigger “dead spots” in my pedal stroke, but the big power quadrants more than make up for it for overall wattage.
I hear about people struggling to hold power on a downhill or tailwind section and that’s where I excel. Holding it on a steep hill, much harder for me. Cadence certainly plays into that, but I think it’s the different biomechanics of climbing that actually results in the lower cadence.
I also think this is somewhat trainable. I’ve always been able to do my best power on the flats, probably because I love training/riding this way with long SS and Z4 work on the flats. The climbs around here are short and I tend to avoid them, so I just don’t think my legs are as efficient when they have to fight the entire pedal stroke. I’ve worked on climbing quite a bit this year and forced myself to do a lot of steep climbing and I’ve seen some improvement in my climbing endurance and efficiency.
I don’t think climb-specific training is going to make a huge difference compared to just boosting your fitness, but I do think you need good mechanics and efficiency to get the best possible climbing performance out of whatever fitness you have.
I’ve got zero science to back any of this up, just personal observation.