Here's what's stupid, though: Mance has a guy on his team, Orell, who can take over birds with his mind. No one wants to have him jump into a bird and go check things out? Even if everyone believed Jon Snow that each castle was manned by hundreds of well-trained soldiers, why not take a peek anyway?
"I could bird warg and be back in like 20 minu- Oh, the new guy gave you some intel? I guess I'll sit my amazingly magical ass in the snow and do NOTHING then."
Maybe Mance is a better community organizer than he is a military tactician, but here's how the dumbshit chooses to handle the situation. He sends the guy who can scout as a bird on (human) foot with Jon Snow, the least-trusted guy in his army. Sure enough, Jon kills him. It's as if Jon Snow's entire plan was to do such obviously obvious things, no one could possibly see them coming.
And speaking of not seeing things coming, Mance's army gets annihilated because Stannis rides up out of nowhere with a zillion horses. You know what could have prevented that? A single bird, glancing casually around the area every few days. It's like Mance met this guy who can Quantum Leap into animals and he thought, "This will be so handy for bizarre sex and nothing else!" What is it with fantasy franchises and their refusal to use birds to easily solve problems?