I agree that a keeper who is not attentive and who is inexperienced should definately not keep two snakes together.
That, however, is a commentary on the skill level of the keeper - not on whether or not this is a bad or unsafe practice.
The things you state as risks really aren't if the keeper utilizes good husbandry practices and common sense.
Make sure your snakes are healthy before you cohabitate them. Seems like common sense to me.1- The spread of disease & sickness
See (1) above.2- The spread of internal & external parasites
Provide multiple hides, multiple basking spots and a large enough enclosure to accomodate them.3- Stress among each other
Inbreeding - Don't house siblings together.4- Breeding/Inbreeding
Breeding - Separate them when they come into season. I can tell when my female snakes are ripening, can you?
They can also become egg bound during normal breeding. Also, how many people have actually had an egg bound snake? How about an egg-bound snake caused by breeding it too young?5- Female becoming eggbound
See answer to (4) above.
Are you serious? Ever seen this in ball pythons?6- Fighting for dominance/mates
We're talking about snakes, not bighorn sheep or wolves. Show me one instance of dominant behavior in ball pythons.
Feed your snakes regularly. Again, has anyone on this forum ever personally witnessed cannibalism between two well fed similarly sized ball pythons?7- Cannibalism
Didn't think so.
Opinions are fine. However opinions based on speculation and second or third hand rumors aren't really opinions.
It would be more accurate to warn a new owner that cohabitating snakes can be done very successfully, but is generally not recommended for new comers because it requires a considerable level of husbandry skill and experience.
Saying that it can't be done and listing a bunch of easily avoided BS second hand information isn't posting an opinion - it's regurgitating bad advice and spreading disinformation.