I figure the main benefit of shorter cranks is getting more aero, mostly for people with issues with their thighs 'encountering' their guts; and they may also help guys with poor flexibility. Pros rarely seem to have these issues when time-trialing, even with "normal"-length cranks.
A while ago, I was reading about some tri guys who not only go for shorter cranks, but will also drop their saddle, even if they lose power, obviously because the net aero gain outweighs the loss of output.
My opinion on the power thing is that it's pretty much a wash: longer cranks may provide more leverage, but the increased hip and knee flexion makes them harder to push when seated, and vice versa for shorter cranks (easier to pound, but may provide a little less leverage). Riding off the saddle is different: the possible extra leverage of longer cranks can be utilized, because the restrictions of hip and knee flexion go out the door.
I've experimented with every length from 165 to 180s (most of which I still have, but I got rid of the 180s. I'm 6ft with long legs), and after all these years, I still can't tell if I'm faster/stronger on any particular length. However, I do like longer cranks for mashing off the saddle on short hills or bridging gaps, but i prefer shorter cranks when my knees are sore.
When i first read about shorter cranks helping with aerodynamics, I rode 165s for a few months, and was no slower than i was on any other length. Uncanny! :p
I heavily persisted with 180s for over a year, but I ended up just hating the position they put me in.
My regular length these days is 170 or 172.5, mostly because they were more ubiquitous on Ebay every time I wanted a new pair of 7800s.