With the over-hung load the shaft goes through a compression/tension cycle each time it turns. Metal fatigue is a complicagted topic and I don't claim to be an expert but have seen some cases with early failure due to fatigue. Steel has endurance characteristics which is the number of cycles it can endure for a given stress level before failure. One good characteristic about steel is that for a given load if it survives about 1 million cycles (only about 8.5 hours at 2000 rpm) it will last forever for shafts that don't have sharp corners of discontinuities. If there even microscopic grooves or defects it can take much longer for a fault line to propogate and lead to ultimate failure. Aluminum is different in that it doesn't have that Endurance Limit. Given enough cycles it will enventually fail. Somthing it think about when you are flying in turbulence and watching the wings move up and down.... I suspect that aircraft designers have taken this into account ;o)