Sarcasm and condescension reeks from your post

Sarcasm and condescension reeks from your post, but I'll answer.

One has a casting time. One does not. One is interruptible. The other is not. I do not think there are many PVE encounters where a Priest is actually interrupted. Ergo it is likely a PVP change, spawned probably by QQing rogues who get dropped from Stealth. wow or Fix would be to make it not hit them in stealth. But that's not the problem. The problem seems to be instant casts.

When a Healer needs to heal a heavy patch of burst healing they unload with Halo or Cascade. But they cannot do this again for almost a minute. So casting Halo and having that moment? Key word. It's fun. Having the spell trashed because of some illusory PVP whine? Not fun. A better candidate for a cast time would be Circle of Healing.