Good Morning All,
I will relate some personal experience on crankarm length: I prefer shorter arms (165mm - 170mm) but I also have short legs and feel better spinning. However, I do have an early Campy Athena (175mm) on an '80's something Bianchi which is only a 53cm frame. I prefer 57-58cm frames (my legs are short but I have a long torso) so I tried the 175mm crank in order to have a "virtual gain" in the seat tube. Do I notice a difference in how this bike pedals from others with shorter crank arm? No! The Bianchi goes up hills well, but I think that is due much more to the wheels (Mavic GL-330's) and tires than crankarm length.
I also ride a Bianchi Cyclecross bike, with 170mm crankarms. This also goes up hills quite well, but I'm quite certain it's because of the gearing :-)!!!
There is more leverage with a longer arm, but in reality, it is a very small amount, and as Sheldon (and others) has pointed out, it is much easier to gain mechanical advantage by changing the gearing ("that's what gears are for...") than the crankarm length. To "measure" the difference(s) in output with crankarms of different length, would require very elaborate laboratory conditions, and to what purpose?
As far as modern "trends" are concerned, I don't know what motivates (marketing?) manufacturers to equip current road bikes with 172.5mm & longer cranks. Perhaps (as Hilary stated) it's simply the current "trend" and will also change in the years to come?
For those of you who are Rivendell members, there was an article in RR 5 ("Cats, Cranks, and Moments") by Nicholas Jasper which might be of interest....?
I'm certain "correct" position over the pedal(s) is far more important than the length of the crankarm!
Malta (A Rainy Day - it's needed!), NY