That's still fulfilling your defensive duties, you're just being more aggressive about it. Artillery generally deals more damage with greater accuracy, and Controllers deal moderate damage to multiple PCs. The point of a Defender is to prevent damage to the rest of the party, and dead monsters can't deal damage. If one of your three hits per round is a crit you can pretty much guarantee that you'll be taking that monster down in a single round.Oh, and for a nice little combo check out the awesome feat: Mobile Challenge. It allows you to shift 1 after a successful hit with Combat Challenge attack.
So, imagine what happens if you decide to ignore your defensive duties momentarily and go on the hunt to bring down the enemy group's spellcaster:
Hit or miss you mark him. So if he shifts away to cast... Bam ... Combat Challenge! And then Shift 1 square after him. Which means if he casts he provokes an Opportunity Attack as well! If he casts in your face: OAtk. If he walks away: OAtk and stop him in his tracks. Fighters vs Ranged attackers = lose lose for them!