Characters are literally bound by the plot, and their power is only as useful as their purpose in the story. Power ups like this Night Warrior are only purposeful for making Tyrande flashy and cool, which is Blizzard's answer to 'making characters interesting'. Genn Greymane? No one cared about him prior to him becoming Worgen. Magni Bronzebeard? No one cared until he turned Diamond. It's become quite cliche at this point. The only difference is Tyrande's form is written to be temporary, otherwise her power levels could have easily scaled up without