Simpler answer
http://www.off-roadweb.com/tech/0112...les/index.html
Essentially a "semi-floater" the axle must transmit the load of spinning the wheel and the weight of the vehicle. In a full floater there is a hub or spindle of some sort that holds the weight of the vehicle, thus the axle shaft is only subject to the load from spinning the wheel. Thus the shaft should break less.