1) A mob has a base 5% chance to crit you + .2% for every level they are above you. Defense removes .04% of that chance for every point above 350. 5.4 / .04 = 135. 135 + your base 350 = 485 defense for a 72 boss. That said, you shouldn't avoid defense by any stretch, it's an awesome stat right up there (and sometimes better than) dodge.
2) Parry is nice and all but Parry Rating costs a large amount of Item Level points. That means it costs much more to get 1% parry than 1% dodge, making dodge preferable.