Cooking oils will go rancid over time, when exposed to oxygen. It takes a bit of time tough. This is why food grade mineral oil is used for lubricating moving parts on slicers, coating carbon steel blades, and seasoning/protecting wooden cutting boards.
That being said, if cooking oil is frequently used until it is no longer viable for frying, it generally won't have time to go rancid. If you'd ever tasted, or smelled rancid oil, you'd know it. I remember as a child occasionally finding a rancid peanut every now and again (peanut oil) and the flavor was truly disgusting, and very strong. It takes a lot of rinsing to get that flavor out of your mouth.
And Bitzer, Though I have been impressed with your posts, and ingenuity as a member of the DC community, your posts on this subject seem almost condescending. Deep frying, pan frying, or even poaching in oil are all valid cooking methods used by people all over the world. I believe it would be difficult, for instance, to make tempura shrimp without frying in oil. I'm thinking any battered dish, be it chicken, fish, or even a Monte Cristo sandwich would not be possible without frying. And what about fried ice cream?
It's true that none of the foods just mentioned are essential, however, they do make life a little richer. So if you don't like frying, then that's your choice, and I'll be the first to support that decision. Just allow others to fry if they wish to, without commenting negatively on the subject.
Seeeeeya; Chief Longwind of the North