Once you roll your manditory (meaning you can't decide not to take the roll) reserve rolls and pass them, but cannot legally place the models on the board, do they not count as destroyed? A similar situation would be when you roll for deepstriking; any models that cannot fit in the deepstrike zone are destroyed. Or for outflanking; any models that cannot fit into the area you have decided to bring them in at are destroyed. Unfortunately I'm at work and can't provide specific references, but this is my understanding of these situations, all of which fall under reserves. Another situation would be disembarking from a transport; any models that cannot be placed on the board within 2" of a hatch are destroyed. Can you think of any situation where models must be placed legally on the board, and if this action is not possible, that said models are not destroyed? If you can't think of one situation that this is the case, I would say every situation encountered when trying to bring in reserves would follow this line of thinking.