I landed on this site accidentally. I'm not a "cook-ie" or foodie; just an engineer who likes to enjoy food AND take care of health at the same time.
The question being discussed is very specific but the answer I believe must be general.
The three physical enemies of all food (indeed of life itself ... I'm not being facetious here) are: Temperature, Oxygen and Light (TOL). All biological matter is made up of complex molecules of C (carbon), H (Hydrogen), O (Oxygen) and (Nitrogen) + traces of other elements. What happens to such natural molecules in the presence of TOL is called cleavage. A cleaver (knife) is an important tool in the kitchen for macro-processing of food. In addition to TOL, chemicals called enzymes can also cleave food at a level which the eye can't see! Cleaving by TOL"" results in the production of two unnatural molecules, neither of which is nutritious; and in some cases may be harmful. Another name for harmful chemicals is free-radicals. In general, food is cooked to remove harmful bacteria and to dilute toxins (most things are toxic only on a matter of scale. In the short run, a little bit does not hurt but a lot does. Examples are tobacco smoke and water). Cleavage happens at the point of least strength (or the chemical bond resistance) of two parts whether it is in the macroscopic (say, a leg of lamb) or the microscopic (say, a "mono- or poly-" unsaturated oil or even a saturated fat such as pure ghee or pure coconut oil or ...). It is "cleaving" that leads to rancidity (acidity) and putrefaction; the products of both are toxic to humans.
"Living nature" wants to live and (though there is symbiosis where no party suffers, I feel it is an exception) at the same time protect itself from harm! Thus the surfaces of all plant matter (including leaves) are protective (i.e. toxic to the creatures that want to eat them). This protection can be in the form of mechanical (skin: of apple, nut: shell, thorns: jack-fruit or any combination) or chemical (natural pesticides, the toxic seed kernels of stone fruits, etc.). Minor (only in terms of quantity) components of oils such as anti-oxidants (vitamin E, an anti-oxidant), (many) phyto chemicals (particulate) are nutritious but easily destroyed in the oil-production (where high heat is used). And then there are vitamins. Some vitamins and many phyto chemicals can be absorbed by us ONLY in the presence of "fat." "fat" = oil!
Since every oil molecule (called a tri-glyceride) is a mix of saturated fats, unsaturated fats (anti mutagen), and the minor components, the minor components are the first ones to be removed by the food companies by RBD (Refining, Bleaching, Deodorizing); all of these processes only degrade the nutrition value to gain aesthetic value. In virgin oils (Olive) it is the particulate components that turn to 'smoke' first (sawdust catches fire before the wooden log does) even at low heat. Once a particle catches fire, it destroys a few oil molecules in its vicinity ... multiply that by the millions of such particles (even in a spoonful) and you can see what will happen ... that is why all virgin oils (and particularly olive) have a lower "smoke point." After that, there will be some amount of toxic component. Our bodies are capable of eliminating some toxicity (free-radicals neutralized by anti-oxidants) but there are limits beyond which it becomes overloaded (you may know the ill-effects of consuming rancid oil).
Deep frying foods is used to impart what is known as "Maillard" browning. The chemicals resulting from browning impart a crunch and aroma (chemicals of browning have their own set of bad issues) but it is hard to escape habit/culture/pragmatism. Deep frying is also used as a water displacement method (say when frying a south Indian vada or a samosa) ... so, a good chef can achieve it by controlling the amount of water in the batter in the first place and overseeing the frying so that the oil is always below its "smoke" point. That is great art/science! Of the common edible oils olive and avocado (high smoke point) are two that come from fruit (and so are micro-nutrient dense); the rest are from seeds and nuts. Ghee** and coconut oil (SAFA:Saturated Fatty Acid) and palm oil a have high smoke point (so good for frying), provide rapid energy (because of their chemical structure) but are not anti-mutagenic (roughly repair at the genetic level - olive oil IS anti-mutagenic) like mono-unsaturated oils (like olive:73%, Avocado:70%, sesame:40%, sunflower:82%, soybean:60%, groundnut:45%, mustard:21%, canola-from rape seed:63%, palm kernel:40% etc) ARE! Therefore, consuming SAFA's must only be in small quantities (frying incorrectly rules against SAFA's ... eventually even SAFA' can be cleaved ... this is where "duration" of cooking comes into the picture)
** I would not consider ghee to be a mass produced oil. Looking at the list of oils above I would say, we must cook (and learn to use wisely) with the oils we have; not the one's that look greener on the other side!