OK.....

There are a lot of issues being tied together here which are not directly (though some may be indirectly) connected:

1) longer crank does not necessarily coincide with lower cadence
2) longer crank does not necessarily coincide with riding a bigger gear
3) longer crank definitely increases foot speed (as measured in distance traveled over time) over short crank at the same cadence
4) longer crank definitely provides more leverage
5) The 'engine' is the determining factor in power output but there may be efficiencies gained by matching crank length to leg length. This should be self-evident otherwise guys who were 6'9" tall would ride 155mm cranks and be just as efficient as if they were riding 195 mm cranks - the guy who owns the hour record is 6'9" tall and used 195 mm cranks on the bike he used for the record. Interestingly, just about all the record holders over the years use just about the same gear ratio but cranks ranging from 170 mm to 195 mm.
6) The most efficient crank length for any given cyclist is difficult to quantify or it would have been done already. My belief is that there is probably a range of crank lengths that would fit most riders and a lot of what folks use is just personal preference.
7) The current 'normal' range of crank lengths offered by the big two manufacturers - 165 mm to 180 mm (with 155 mm to 185 mm being available from smaller manufacturers) - is more a factor of frame construction constraints than human body constraints.