Personally I think the whole charging for luggage is a con, think of it this way ....
Skinny bloke going away for a couple of weeks with a large bag so he pays for his ticket plus extra for hold luggage.
Great big fat bloater (*) going away for a couple of nights stuffs a pair of clean under-crackers in his pocket and that's it.
They're sat next to each other on the flight out yet the one paying extra because of the weight affecting the flight is actually 10 stone lighter than the chap in the next seat ... it's a con, it's all about extracting additional monies from people they know have no choice but to pay. Whilst I can appreciated the business model these companies follow, anyone daft enough to think what they pay will be the "headline price" is in for a shock.
Won't be long before these airlines will be weighing passengers and charging prices based on your own weight!
(* before anyone gets offended - I'm a big fat bloater
)