Really? Where did you read that? Everybody here will tell you it's against the commonly-accepted wisdom. I'd love to see a reference.
I actually do have an enormous tank full of sub-adult females here, and a second large tank with the off-duty breeder males. Everything has been perfectly harmonious for 2 years, with the following caveats:
1.) I have to take them out of the tank and feed separately every week. This is non-negotiable. The risk of feeding 'accidents' if two snakes go for the same mouse is simply too great.
2.) Housing three of four girls in one tank means cleaning that tank 3 or 4 times more often. Once a month I have to clean the hides, because they're gross. Between this and feeding separately, I'm losing in time and effort what I'm presumably saving in housing costs.
3.) I live in the desert, so keeping the humidity in a good range for shedding is difficult. Tubs work much better for this than the big tank, and I've actually begun putting the bachelor/ette snakes into my spare breeding tub whenever I notice they're in shed.
4.) The males in the bachelor tank have been locking with each other the last two winters while they're waiting for their turn to cycle in with the breeding girls. This doesn't seem to be a problem, but it's, uh, embarrassing when company comes over.
5.) I won't house (or breed) ball pythons together if the largest weighs more than twice as much as the smallest. Even if there's realistically no real danger of aggression or cannibalization, the big one could still squash the little one. I just can't convince myself there's no danger.
But that's it. I'm one of those aggravating people with multiple females sharing an oversized 60 gallon tank whose animals eat really well and are healthy as horses. Every night two or three of them come out and climb around, and I swear if one takes up a project (let's poke at this corner all night tonight! Poke! Poke!) the others will decide it must be the thing to do. I do think they get more exercise with this set up then they would in separate tubs. And it's frankly a lot of fun to watch them.