I do believe that the drains get plugged with debris easily. Also, zealous undercoating appliers often plug them up.

A vent at the top could be a good idea if coupled with regular clearing of the lower drain and (at least) a decent paint job on the panel. It seems that once moisture gets in it evaporates and condenses on the panels causing them to rust faster, especially if the pait was not very good in the first place.

Good painting after a repair ought to do the trick without having to punch more bungs in the chassis.

Mike!