It's bad if the gerry can ends up full of water; depending on how old we're talking, too, older gas tends to lose its flammability, which is kinda crucial to the "combustion" portion of "internal combustion."
Also, it's bad if the gerry can turns out to be full of kerosene, like I almost did one time.
Yah, more than six months means its nowhere near as combustible as fresh. Also condensation can form in the jerry can which gets into the gas. If your car broke down, that's a good sign that the gas was bad.