Cool topic, Garth!
I'm kinda partial to my Sugino Mighty Competition and Super Mighty cranks: they've got 171mm arms, and I suspect that a lot of Japanese bikes were fitted with them during the 1970s.
I'm guessing that bike companies started thinking ergonmoically--and started using mid-length crank arms for the medium frames--sometime in the mid- or late-80s; didn't Shimano's Biopace stuff come out in the late 80s? If you need a larger frame, you probably have a longer femur, so the 175s would be fine (at least I'm thinking that femur length is the relevant measurement here as it relates to leverage and such); but folks who buy in-between frame sizes would probably benefit from the 172.5s.
There's got to be a rationale for the longer crank arm lengths (180mm) on mountain bikes, but I don't know what it is.
It'll be fun to see what insights knowlegable folks have regarding crankarms. . . .
nath 171 dresser