Pro-rating is a dangerous game to get into when the player never plays a full season
Equally dangerous is the "I'll just pick someone up off the WW" argument, not sure what kind of leagues you're in, but in mine there aren't 40 pt defensemen lying around to be snapped up.
Also I don't know how many IR slots you typically play with, but in my leagues it is 1 or 2 at most, so having a guy constantly filling it makes it hard to use it for anything else.