Colter,
Your questions probably should be directed to Dave Field, a long time active maintainer in Maine, and past chairman of the ATC.
Dave and his family built and still maintains the Popular Ridge Leanto. Remember this one with the baseball bat decking?
Anyway, he maintains a book at the shelter titled “Why we do things this way we do in Maine” or something like that. The horizontal knee buster log in the front of shelters is called a “deacons bench.” If there’s an answer to your question, it should be in his book. Like you, I have been told that they’re there to keep the porcupines out, but as I have learned, it doesn’t. From a structural standpoint, a horizontal log like that does serve to hold the lower front corners of the sides in place and prevent one corner or the other from walking in or out over time.
About all the I know is that the “classic” lean-to design probably has its origins when the trail was first built in the 1930’s by the CCC.
So, I’ll ask, if someone knows why there is the knee breaker deacons bench, please enlighten us.
Peaks