I generally agree with your point, Draug, but how does a defending fleet split its defenses across two coasts? Of course, one could imagine that land-based support staff is stretched, or that morale is drained by the other coast being pillaged, or that commandos are dropped to move toward the other coast, and so on, etc.
However, Diplomacy is a very abstract game, and I think the rules are and necessarily must be more motivated by practical game play reasons than realistic war simulation. At some point, one has to make these choices in abstraction. For example, consider the issue of two fleets trying to switch places like this: Por -> Spa(sc), Spa(nc) -> Por.