In reply to Jim Hamilton:
It is a bit simplistic, it's intended as a general rule to encourage good practice.
You shouldn't plan to be lowering off, but if you can't get up the route then it's better to lower off than scrabble about getting hauled up by the rope!
You should always rig the krab so that the rope won't rub the rock if it goes tight. On some routes this can mean that the krab is so low down that you are a decent way above it when topping out, which on a rounded sandstone mantle can be pretty scary. In these circumstances we will sometimes place another krab at the top that we'll clip when we reach it.
Ideally you shouldn't be attempting routes that you don't have a good chance of finishing, but we all have projects and for these we take extra care to ensure that lowering off the crux will not result in the rope rubbing - even if this makes the top out scary.