A lot of people blame IBD for deaths that are a result of poor husbandry and many other things. Like katalyst said people are way too paranoid about it. From what I have read, IBD can only be diagnosed by doing a necropsy, how many "former" snake owners do you know of that have had that done by a qualified vet?
With that being said you still must quarantine snakes before introducing them into even the same room as your other snakes. Mites, other parasites, or many other things can be passed to your other snakes from a new purchase. A minimum quarantine period of 45 days should be observed, but more is a LOT better. Some people even recommend as much as 6 months!