Seeing that picture of the girl in the link just makes me shiver.
I wont ride without leathers full stop, unfortunately i learnt the hard way when i was a lad, went out in jeans and fell off resulting in some nicely gravel rashed legs !
My other half is a stickler for me wearing leathers. Shes an A&E nurse, so has seen far too many injuries from people riding with shorts, tracksuit bottoms or jeans on. Albeit she only comes out on the bike once or twice a year, she still has her own leathers and always wears them .... Plus, it makes her butt look lovely !
And lets me honest, leathers aren't all that expensive these days. You can pick up a half decent set of leather for around £200-£300. Yes they may not be the most protective, but are a damn sight better than jeans.