I will certainly agree that shoelaces are rather too small to work well as prussiks. They can be rather challenging to manipulate even with proper diameter rope. However, to the best of my recollection, I was reading this in an AAC accident publication (a while ago)and it actually happened.

In a more common scenario, I wouldn't hesitate to use my shoelaces to lash a shelter together. In the morning, unlash, retie, and start walking. However, it is awfully easy to simply carry a decent supply of cordage.