My sympathies for anyne who has had the misfortune of being mugged or almost being mugged.
Yeah, the other big issue with handheld cases is the potential for it to attract the eyes of thieves. Especially important looking cases that look like you are carrying around stacks of 100 dollar bills and some uncut cocaine.
With a backpack, a thief would have to cut the 2 straps or force you to hand it over. Even a messenger bag makes it harder for someone to grab and run. And it's less attention drawing.
Personally, I don't even like the fact that my Targus backpack is labeled "Targus" because the brand is known for making laptop cases. I am thinking about switching to a less known brand. Protecting your $2000-$4000 investment isn't solely about having enough foam to shield it from drops. It's really about overall peace of mind.
I think "SOME" people get these metal/graphite cases more for attention rather then for protection. Or they've seen one-too-many spy/action movies. Yeah why not get a trenchcoat, some dark sunglasses and a pair of handcuffs to finish off the look.