That's why its imperative to use hardware cloth on your coops and runs. Even weasels and snakes can't squeeze through the 1/2 or 1/4" inch mesh. And of course, make certain that there are no other small openings anywhere in the coop structure.
I don't know if I'd say it's overall "imperative" because depending on where you live, it may not be. I agree it's nice to have but some people have very large coops and hardware cloth becomes cost prohibitive if you're looking at a large setup for most folks.
People have been raising chickens without hardware cloth for a long time
