Because aggro is so fun of a mechanic when a tank is undergeared so a unpredictable crit from the DPS yanks aggro.
If you think threat is going to make it "more interesting", then I'm not sure what else to say honestly. No one is going to be like "Man, I sure am glad I have to lower my dps because our tank is undergeared". They're just going to start screaming at the tank to hold aggro.