When the arena starts your 3x Healing Spheres are still at arena entrance but you are not actually spec'ed in to the Healing Sphere talent(check picture).
Pros:
Might be useful when you are in full cc and you play with a class with good mobility that just runs through the Spheres and tops himself up to max hp.
Cons:
Healing Spheres expires and disappears after a certain amount of time(I don't know how long but about 3mins I think).
How this works:
Remember when you have CD on a talent and you can't switch to another one before it's out of CD. Apparently when the CD is stacked like healing spheres(3x 15sec CD each). It's enough to have at least 1x of those 3x stacks out of CD to change your talent. Also the Spheres doesn't actually clear from the ground when the arena starts. This might be useful with other spells with similar function, or maybe spells that will be introduced in Battle Of Azeroth.