It has been said a few times already on the Forums - the low fares advertised by Ryanair, Easyjet etc are no more than very clever marketing hype!!! ".....There's no such thing as a free lunch!"
Excess baggage charges are very lucrative for MOL and his pals.
I agree with slim slag's notion that its duty-free bottles, coupled with pax' lack of discipline and self-control that leads to injury. However, no-one will ever convince me that the FAA permitting US carriers 18kg per single piece is safe! 18kg falling from an overhead can and will kill you and, although overhead lockers are strong, the catches on the doors are not!!! Anyone who has seen these monster US carry-on bags fly from an overhead would agree! The trolley-wheels, as individual pressure points, are absolutely lethal!
The deaths from the British Midland disaster at Kegworth (M1) were mostly caused by cabin baggage and cabin detritus - and British Midland only allows 6kg - the same as British Airways or most CAA controlled carriers. As BA ground staff, I will "turn Nelson's blind eye" to up to 9kg, but for the safety of other pax will ensure anything heavier is checked in. (The Club/First allowance of 18kg is in two separate bags, neither of which should exceed 9kg!)
"Good Customer Service involves the consideration of all the pax on an aircraft, not just the individual in front of you!" BEA Training Course 1967.