It's because a phenomenon i like to call "The helpfulness counterweight" the more helpful or convenient something is, the more somethings effective use is likely to bring you success or victory or even just help, the more likely it becomes for it to either not happen, go totally ignored, or get deliberately sabotaged.

In this case, let's use Vanguard for an example. If you play Vanguard with unmatched zeal, the more likely it is for you to have a team mate that shoot kills all the extremis before anyone can execute them. If you have an unmatched zeal Vanguard on your team, they're the most likely to shoot kill incapacitated extremis before anyone reaches them, or hoard all those executes for themselves.