People are going to house their animals however they want. If they ask me, will explain why I do not house my animals together. However, I do not tell them that they are wrong. You have to remember, just because someone does something differently than you does not mean that they are doing it wrong.
Some species do better than others when housed together. Anytime animals are housed together, however, the owner should watch them more carefully for signs of sickness or stress.
I have sold to people that house their snakes together. I always tell them my thoughts on the matter, and let them make their own decision.