The list of cons is: Possible stress, the guaranteed transfer of disease, if one gets sick, the other is sure to get it too, if it's communicable. On that front, with multiple snakes to a cage, it's more difficult to monitor changes in your snakes, like who shed or figuring out which one regurged. Then there is the possibility of too early breeding, which can cause lots of problems for the female if her body wasn't actually in good enough shape for it and that could range anywhere from requiring vet care to her dying from it. A male kept with a female always might stress her out enough to cause health problems in one or both snakes by trying to breed her too much. Two males together might well fight with each other constantly during breeding season, which is likely to be even worse if there are two male and one female in a cage.
Plus corn snakes can be cannibalistic and sometimes do eat each other. That might be fine for the cannibal if the other corn is a lot smaller but relative size doesn't seem to be a large factor in one deciding to eat the other and a snake that eats a similarly sized cage mate almost always regurges and then dies, killing both of the snakes.
Even in snakes that are considered social, like garters, and cannibalism is almost never seen in them, keepers still have to take into account all the other possible problems listed up above and try to mitigate them. Now garters have had a few studies done on them that suggest they get some kind of emotional benefit out of being cohabbed in some cases, so sometimes it might be worth it to cohab them. Most other species out there though have never had any kind of benefit to them proven that might make it worth any of the above risks to cohab them, any of the proposed benefits to the practice with most snake species actually comes down to the convenience of the keeper, not the snakes.
