Three words.
Cascading
Defense
Failure
Without DDR, high-defense characters basically become glass cannons waiting to happen.
Yes, if they ALSO have high Resist that takes a while.
But if you are in combat and suddenly your defense goes from a floored 5% chance to hit to YOU GET A HIT! AND YOU GET A HIT! AND YOU GET A HIT! YOU ALL GET A HIT! It's a setup for WILD swings in performance and efficacy.
This is why, on builds with no native defense (and no DDR), I generally caution people to treat it like ablative armor (the way Absorb shields are treated).
Without that DDR, the ability to build high meaningful defense is crippled. As late game starts chucking debuffs hot and heavy. Additionally we start seeing increasing amounts of incoming damage from things OTHER than S&L.
And, having done the "Piss 'em off and stand there" maneuver testing before. Yeah. It's pretty damn neat.