The three piece crank gives you more option of spindle length- sick bikes has extra wide spindles, but usually a fairly normal length will clear a china girl with no pull start-
You can use a lighter crankset then too, and maybe even some titanium spindle bolts- it all saves weight, if you want to keep the bike lightweight.
There are actually about three designs- a couple of which use the smaller 3 piece bottom bracket cups and bearings, and one that uses the larger single speed bearings and locking cups- the latter doesn't give you an option for choosing the spindle length though.
If you are wanting to keep the bikes weight down, and get a really easy pedal- but don't want to change the one piece crank, you can also get a really small sprocket- like 25 or 28 teeth, and that not only saves a lot of sprocket weight, but you then also cut the chain length way down as well- like 10 or 12 links- a substantial weight savings.