I'm probably a guy who almost never WDs. And the perseverence does pays off. I remember once long ago when playing Whistler I made 38 front and then 27 back, which gave 65 and the 9-10th place, if I remember correctly :) I know it sometimes sucks to play knowing the best you can get is the money back or even the 50%, but I believe in not giving up prematurely...
The only rational reasons I see in withdrawing are either if there is no realistic chance to even get the 50% back or if the max possible return is 50% AND the course is very tricky with high chance to lose balls...