Over the years I've tested myself on some ridiculously steep climbing, both on and off-road; I have pretty good balance, but in this case, I don't ever remember it being an issue. I definitely used the 42 on the 17 percent section before the visitor's center and then any of the steep switchbacks on the gravel. I had to walk, once, on a steep washboard dirt section that was pretty soft right before the gravel ended. I could not power through it after six hours at nearly 12,000 feet. :) It was only about 15 feet. I had read about people who did it on 38mm tires with 30x34 and I was not going to attempt that. IMO you would need to be able to push at least 200 watts at9,000-12,000 feet to get through the gravel that way.
I was doing about 150 watts or so at that point (Stages right arm meter which reads 10-12 watts lower than my powertap).
After the dirt ends, there is a tiny break and then it climbs up through some brutal switchbacks at 10-13 percent; on the day I went there was a 10 mph headwind, so it was the only way I could breath and keep moving forward.

Also, I was running a 29x2.2 Speedking on back, so that is not quite as low a gear as on a road bike.

Riding at 3mph on a flat surface is awkward.

But it's much easier up a steep hill. Sequentially putting force down on one side then the other is part of how riders balance at low speed: in each pedal stroke, you counter and reverse the instabilities that you were left with after the last stroke, rinse and repeat. It's usually not a conscious process, but rather, what the brain does when it knows how to ride a bike.
On a flat surface, this is hard: if you gear low, putting enough force on the pedal will cause you to accelerate, and you're no longer at 3mph; if you gear high, the balancing cycle happens slowly and you might need to do more "conscious" balancing.

I've never really thought about that element of climbing at slow speeds. I went back and looked out of curiosity because 3mph sounds really slow. Sure enough, I average between 2-3 mph on the section right after the gravel ends where it averages 12.2 percent for 1.5 miles (and where I would estimate, there was a 10mph headwind). It tops out at 13.8 in one part that is at 12,225 feet. I remember dying a little at time in that section just trying to turn over the 24x42. From the gravel to the top, I barely managed to average 135 watts from fatigue and/or altitude. The last 1.5 was "only" 8 percent and there was a tailwind at times, so it felt almost easy.

The real problem for 99 percent of the people (leaving from the US or Europe) is that you can't simulate riding at 12-14,000 feet unless you live at the foot of Pike's Peak in Colorado. There is plenty of stuff in the Andes, but no one can take a month off to go train in the Andes, so you just suffer past 10,000 feet.

I remember thinking at 10,000, "Wow, this has been great! I really prepared well." And then the last 3,800 feet made me suffer.

I used to ride a 54, 42 and a 11-18 7 speed straight block cassette, as 175mm cranks. Now 53, 39 and a 11-26 11 speed assette and 165mm cranks. Me? 6’, 157 lbs, 67 years old with a 87.8 inseam.

Back in the mid-80s, I had a 52/42 with a 6 speed 13-24 freewheel on my road bike, and my first real tri bike in '91 had 53/39 with a 7 speed 12/21 cassette. My first MTB had 48/38/28 with an 11-28 cassette. Now, I've got 50/34 with 11-34 on my road bike, 50/34 with 11-30 on my tri bike, and 32 x 11-46 on my MTB...

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"I'm thinking of a number between 1 and 10, and I don't know why!"

Like most of y'all my 1st "real" bike was geared for a tour rider. I had a semi-beam bike called the Mongoose RX 9.7.

Came with the usual 53/4_? and of course 11-23. That served me well for a few seasons before getting a Softride, which I eventually installed a triple with 11-25.

Fast forward to my latest bike that I got last season an A2 SP. Custom ordered with a 50/36.... and 11/36 on both of my wheels.

Having such a doable gear actually makes me able to ride hills more aggressively.

I can stand/grind whatever... really crush my legs, knowing that I have a bail out gear that I can easily turn, no matter the grade.

Served me well in Nice last fall =)

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http://www.fitspeek.com the Fraser Valley's fitness, wellness, and endurance sports podcast