Why isn't the entire Orbiter's belly ONE PIECE??
So that if one tile is damaged it can be replaced. Can you imagine the budget problems if they had to replace the WHOLE underside of the shuttle after every flight? Also- a cracked tile does not instantly spell death for all aboard. The real problem will be if the crack was determined to be past TOLERANCES. Anyone who has worked on any kind of aircraft can tell you that they fly with missing fasteners, cracked skins, leaking fluid all the time. It is about the tolerances, not the actual defect.