Ideally quarantine will keep the new critter completely isolated from the main collection, with no possibility for cross-contamination of equipment, feeders, water, or air/HVAC. While most reptile diseases aren't thought to be airborne, little study has been done on their persistance on surfaces, so IMO it's better to err on the side of caution.
Most people don't have the resources to put multiple structures onto their property so they can isolate new arrivals. I manage by having a friend keep my new acquisitions at his place for three months before I bring them home, as his wife doesn't like snakes but will put up with 1-2 at a time in the house. She figured out pretty quickly though that once QT was complete and the snake came to my house, it wasn't long before another snake was brought into her house...
I've also seen accounts from people who have had BPNV, OPMV/Ferlavirus, or something else (usually unidentified) rip through their collections of pythons and boas; is there a disease in colubrids that causes the same levels of devastation to a collection?