Oh wow, that first link looks SHORT! Granted, her coat probably hiked up her torso from her arms being stretched out but still, looks a little short, even for me!
I think I'm going to agree with ErikaLynn on this one though... I"ve always thought the longer coats looked more frumpy and less tailored, like there was just too much loose material flying around! Plus, don't you accidentally sit on it when you're posting? I have a winter coat about that length that I rode in for a while and it drove me absolutely nuts when I would always be sitting on the hem while I was posting. I'm not saying I love showing off my hiney, but usually when I"m schooling I'm wearing a more fitted polo/tshirt tucked into my breeches so.... even a short coat is going to cover up more then when I'm schooling!