Usually from what i have seen, Were cards havent been returned, the players has had a terrible round, had to leave the course or injured themselves.
If you were to put a N/R for a hole on stroke play, i would think you would be disqualified for not finishing a hole.
In Stableford it is considered good etiquette to pick up and put n/r for the score when you can no longer score any points.
In terms of handicap protection, its usually the unscrupulous players protecting the handicap from going down not going up and if you had such a terrible round and didn put your card in you would stopping yourself from getting 0.1 back on your handicap.
I carnt see many people playing very well in a medal competition (say 4 or 5 under there handicap)and being in with a shout of winning and not handing the card in to protect their handicap.
But then again, i dont know some of the lenghts people will go to stay off a higher handicap