Hi majorgator,
If you haven't done this test it may seem like so much voo-doo but if you've done one, it becomes apparent the usefulness of this test.
What your reasoning revolves around is that the test conditions somehow alter the construction configuration of the trailer, creating abnormal sealing conditions that aren't real-world conditions. That isn't the case. The pressure inside the cabin should be increased to a level of 1"-2" of water column. This only allows for the pressurized air inside the cabin to exit out of any existing holes. It doesn't force open any sealing surfaces or bend any structural elements. Yes, seals around doors or openings within windows will pass air but that's not the objective. The purpose is to uncover any holes in those sealed surfaces that we don't want to have water enter. Places like the joint between roof and wall, around window jamb flashing, in corner joints where siding breaks occur, around any roof through holes.
The bottom line is I like my rig sealed where it should be and not open to any water intrusion, YMMV.