Somewhere here on RC there's a picture of a Yellow Tang with a broken rubber band hanging out of it's rectum. Remember that rubber bands degrade aver time (fairly short amount of time) and will break. If the broken band is ingested by a fish that thinks he's eating a tasty treat, the outcome could be pretty bad.
I don't remember if the Tang in question passed the rubber band safely or not, but for that reason, I like to keep them out of my tank.