Yea, and the prophet Lore looked upon his apprentices and said unto them;
"(540) Defense isn't really a goal or an end that you should reach - it's not a peak - it's a MINIMUM"
Check Vene's site at TankingTips.com - A guide to tanking as a Warrior in World of Warcraft for the 'How good is defense' posts. Defense gives almost - and I mean almost as much avoidance as dodge does - even after reaching 'crit immunity' at 540 Def Skill.
You need to refer to Crit immunity, not defense Cap. Cap implies that after reaching that 'Cap' you gain no extra benefit from that stat. Which is wrong in this case.