The reason that some people think a BP can live for months or even years with IBD is because very rarely does a healthy, normal BP get tested for IBD. They only get tested for it when they start showing symptoms.
So, let's say my Mercedes has inclusion bodies (knock on lots of wood). But I keep her temps and humidity good and she doesn't have mites or get an RI or become exposed to anything else. The inclusion bodies don't make her sick, or kill her. It's the stuff she catches AFTER she has inclusion bodies... her immune system can't keep up and she dies quickly as soon as she catches something.
This is all speculation... no one's done enough research on this to be sure, but this is why people argue that a perfectly healthy snake can have IBD.