Lids... One thing I've noticed several times is that when they are mentioned its all about heat retention (which is very important). However you read another thread and it's all about the procurement of water.

If you only have a limited supply of water, lids are also good to stop all that precious water from evaporating. May only be 1/8th of a teaspoon at a time, but in serious conditions that can add up. So even if your lid is just tin foil with a dimple in the middle I would still want the cup covered.

My .02