Here is my "travel bag" for my AK, a ~$20 tennis bag from academy. As you can see, an AK with a folding stock and 30 round mag fits perfectly in the bag.
In addition, I can wrap up my surplus Chinese chest rig (or as Andy calls it, my mag bra) and throw that in the bag too, though a gun plus 4 loaded mags does make it a bit heavy.
That being said, it provides a lot of options that a fixed or collapsable stock cannot.
In a katrina type disaster, I can look like an idiot who couldn't bear to part with his fancy tennis racket, when in reality all I have to do is unzip the bag and anyone planning on robbing/mugging/looting on my property would be in for a heck of an unpleasant surprise. A tennis racket bag is also less likely to draw the attention of the police than say, an AR on a single point sling strapped to your load bearing vest.
If you'd rather not have all your neighbors know that you own guns, transporting the gun to the car in a tennis bag for a trip to the range is a far more discrete option than a long and obvious rifle bag. (although, a golf bag might fit the bill there....though a heavier and more expensive option)
Just thought I'd expand a little on my "folding stocks are good" statement earlier.
