After seeing the end of "Reunion" and the clip from the upcoming episode "Beyond the Wilds", it got me thinking: What if Kuvira's decision to cut down the giant Foggy Swamp tree triggered a war between Man and Spirit? What if Kuvira's actions might forever scar this world?
The Avatar is there to bring balance between the spirit and human worlds.
Also the spirit vines were doing a number on Kuvira's troops. Yes they were winning. but they were wining expensively. (ie their losses were great). In the end Kuvira wil get what she wanted. high energy vines.
Also remember they showed us the big red stop button. The Avatar can drain the spiritual energy from the vines from inside the spirit world. Korra can litterally drain away the explosive energy thus rendering the bombs little more than vines in glass tubes. It might be a bit more complicated than that.