If you are playing 2s 3s or 5s as a healer, you are going to get CC'd a LOT. Ysera is still active whilst being CC'ed. It has not LoS requirement. It only has a total range requirement. If you are playing versus good players, you should realistically expect some damage to flow most if not all of the time, which in itself describes ideal conditions for Ysera. Now add to that the fact that it MIGHT be that single tick of healing needed to save someone whilst you are in a blind or cyclone, and you have a tactical advantage.
Burst healing is NECCESARY when you are reciving burst damage, but the problem with implying that you can try to sustain ANY kind of rotation in a PvP enviroment comes from someone who never actually has tried. On top of high damage cooldowns, teams will try to add in stuff like phasing, or hard/soft CC on healers in order to ensure a kill. Enjoy having Cenarion Ward off cooldown and ready for burst healing if your teammate has been death gripped LoS; is in a smoke bomb, or you are incapacitated.