Yea, it just makes the AI block more. On the highest they are very bad about chamber blocking you.
As for the behind them thing in sieges, maybe it depends on the situation - I just tried it in the latest version of warband and I was able to cut down 30 or so of them one at a time, I got one or two turning after me but the huge mob on the wall just focused on attacking my huscarls on the ladder, even though only the first couple of them were actually able to swing at them. The "smart" thing to do would have been the 10 or so nearest the ladder keep focusing on that while the 30 or so behind them turn and rush me, but that never happened - at most I had 3 of them turn on me at once, and just by backpedaling and whacking on them I killed them then went right back to hacking the rest in the back. I took more damage while jumping over them (I got the weird thing where you jump and kinda bounce over them and slowly float above the mob off to the side, and got whacked a bit on the way) than I did the entire time fighting them.
It could be that the large amount of huscarls I had trying to get up the ladder (I tested it with a save where I had 100 huscarls, heh) were drawing their attention, but even if that's the case it really should prioritize mobbing the guy slashing their troops down from behind over rubbing against the friendly soldiers in front of them in a futile attempt to reach the ladder and get a swing in at the guys trying to come up.