I think it's a matter of resources. There is not a lot we can do for these elves, in all honesty. The best we can do is try and give them a chance to escape the battle while we keep the draconids focused on us and our allies. It may not be possible to both win the battle and save a significant number of these elves. At this point, I think winning the battle is much more important in the overall scheme of things. The odds are not in our favor. We need all of our resources focused on the defeat of this army.
Opening the gates has a good chance of reducing one of our main advantages, namely fortifications. We can't have our guys on the wall worried about enemies on the inside that got in while we tried to save some elves. We need them focused on defending the walls.
A stand by the heroes to buy some time is a poor option, but may be the best we can do, especially if we draw our line at a point where we can be supported from the walls. Even if we had some sort of signpost, there's no guarantee that these starved, terrified elves would even follow them as chaos overtook the field. Medieval battle is a notoriously chaotic beast as it is and trying to impose order on it will only take you so far.
We do what we can and the elves may just have to take their chances. It's a tough situation, but our primary goal has to be the defeat of the draconid army.