Main reasons not to cohab:
1. Undue stress. Snakes are solitary by nature. Stress can cause any number of health problems, including not eating and susceptibility to illness.
2. Breeding too young, if they are male and female. Many people don't know that snakes can and will breed when the female is too small, which can lead to egg binding and premature death.
3. Possible cannibalism.
4. Potential introduction of disease and parasites without proper quarantine.
There are many reasons why not to cohab. I think many people do it because they can't afford a second set-up or don't have space for it. I don't feel its worth the risks.