It's not that ignoring a mechanic to gain DPS is a downside, it's that (when able) it encourages you to use it for DPS more than survivability; since survivability can be made up through other means, while DPS cannot. Warriors faced a similar concern in MoP wherein they could choose between using Defensive Stance to mitigate damage or Berserker Stance to gain DPS. It sounds great, who doesn't want more damage? Unfortunately it encourages selfish, personal play.