The question is here is whether the "punishment" or cost is worth the reward. Consider from a psychological perspective the following:

(1) taking a tanking stance before engaging enemies. The cost is dps, the reward is enmity or control of the enemy plus a reduction in damage taken. The reward outweigh the cost.

(2) building 100 rage while in tank stance and then swapping to dps stance. The cost is increased damage taken, but what is the reward? The reward is increased dps, BUT with the loss of 50 rage, and rage being units of measure associated with dps, it FEELS like you're also losing out on dps-at least in the short term.