Soothing mist generates chi, spending chi generates mana tea, and c'mon soothing is not so expensive spell. Spamming spheres is not always easy, you can miss the target causing his death. Soothing/enveloping mist is safer if you take this into consideration. But I agree that for selfhealing, spheres should be around 90% of your healing. But usually arena opponents are smart enough and don't hit MW so there is no need to heal yourself in arenas. Another story is RBG especially when you carry flag as MW ( I do that). Spheres save your life and are impossible to interrupt.