Baggage handling type people don't have to haul you around and stow you beneath the plane. I was recently bitten by an overweight bag, and subsequently broke it into a second carry-on in a return flight. 25$ for extra bag, 100$ for overweight...

Anyways, what I was traveling with was unreasonably heavy, and would be a hassle to toss into the stowage, probably be a greater burden/wear on the conveyors etc.