I think a longer crank does give more leverage, all else being equal. But if you don't have the longer legs to push it, you'll strain and need to shift to a lower gear, cancelling it out.

It seems that crank length isn't a big deal for the vast majority of people since so many other things on the bike (leg extension, saddle fore-aft, etc) can be adjusted around it to get reasonably optimum power output and comfort. It is too bad that the larger companies don't at least take a token interest in the outlying sizes, though -- making smaller runs of cranks like 155mm or 190mm wouldn't directly help the bottom line, but loss leaders like that can help with goodwill and winning customers over to your brand.