For this past winter I changed all my unis (29, 26, 32, 36) to 150’s, then when the weather warmed up and the trails started drying out I switched back to 165’s.
When I went down in crank length it was a compromise, I was trading increase climbing power with improved spin, but in the winter everything is so wet that most of my rides are flatter and less technical. Then, when Spring came and the trails dried out, I wanted my climbing power back, and so I swapped all my cranks back to 165’s.
I do tend to use my cranks for braking when I run 165’s vs 150’s, though I still use my brakes a fair amon for coasting grades. It seems like brakes throw my balance off more when I’m running longer cranks, but then when I run longer cranks I don’t need the brakes as much…
My climbing is clearly better with 165’s, with the longer cranks I can clean sections that would be a struggle or not possible with 150’s, but with 150’s m ride is smoother and takes less effort.
When I rode with Killian at Antelope Island, I was borrowing a 29er with 165/137 cranks. At one point Killian swapped his cranks to the 137 position, a while later I did the same. I wouldn’t say it was easier with shorter cranks, but I was feeling really “wonky” with long cranks after having been riding 150’s for the past six months, so though the 137’s were short they felt closer to normal.
If I was riding trails that were consistently graded and no technical, I’d probably run shorter cranks, I mean why not, a longer crank would serve no significant purpose, but where I live it is very technical and there is nothing graded about our trails; they go up and down at the same time 
My ideal cranks would have three positions: 165, 150, 135.
