I read a post on another site about the importance of defense and it made a lot of sense so I made it a personal quest to make sure I had a healthy defense.
I made level 6 buy giving 120 bone chips to the dwarf at the Paladin guild in Kaladim to save some time. So, when you make level 6, go ahead and continue with the rest. I wouldn't advise passing level 6 before you start trying to AFK train your defense.
Run around til you find a single cht'thk bloodling that cons blue, run over by the PoK book (if he kills you then your corpse will be right by the zone in) and let him beat on you while you play your Hymn of Restoration. Go eat lunch, wash your car, spend some much needed quality time with your significant other, or do whatever... come back in a while and your defense should be trained up max, which you will find EXTREMELY useful later on. Make sure its a bloodling or something else just barely blue still, as they wont be able to damage you enough to kill you with your hymn up. You shouldn't die with only one on you, I only dipped to 96% once. About a half hour later I was maxxed on Defense. It has made training my defense up since then INFINITELY easier. My poor 29 Pally never got his Defense trained and my now 20 bard has more defense skill than him. Hope someone finds this useful.

I agree that eventually your defense skill will catch up, BUT at later levels when it has fallen behind and you're participating in groups with real tanks or start kiting everything, you will rarely be getting attacked directly. Therefore, it will be harder and harder as you advance to get your defense to budge. The only thing that keeps you from being hit is defense. AC reduces the damage from attacks and agility only helps increase defense skill and make dodge, parry, and riposte skills work. When you hit level 40 and your defense is still struggling up, you'll be getting whacked for around a 100 points a shot, and probably getting smacked every hit. At level 20, I had my defense up to 90, even after being PL'd from 14-17