My proposal, then, is for a couple of changes to the system that make it feel less arbitrary without changing it drastically. Some bridges should still be collapsible (some won't be, because they might have been properly reconstructed or whatever), but I don't think the Swimming stat should be used to calculate that. Instead, a Perception/Traps/Hazards check should be the governing stat, and if one of your party members sees the weak points in the bridge, they should be highlighted somehow. Then we should be able to skillfully maneuver the characters across the bridge and past the traps. Assuming we fail at that, then the Swimming stat should kick in. Or perhaps the Climbing stat should kick in first ("Mraculously, Dave manages to catch a firm hold and pull himself up!"), then Swimming.
What do you guys think? I am ready for the inevitable dissection.