Aha! You may be a little mistaken on its capabilities.
First, the Orb is single-target only, already making less strong versus groups (which is what Tome of Pain can achieve quite well, and both Merlot and Furor can assist in that as well, if you’re using a piercing Wand).
Second, the Range is only 6 tiles, which isn’t that much. Sure, it’s about Ray Katana range, but you’re still a priest - you don’t want to be close and up front with a target unless you’re spamming Prot.
Third, although it does swirl in place, it’s relatively stationary. Any moving boss won’t get the full amount of damage from a single use.
Fourth, the damage is spread over a pretty long amount of time. 6 seconds is quite a long time, especially given that most bosses tend to have some invulnerability going on.
Finally, its healing potential is severely hampered. Sure, you get Healing, but that won’t save you from imminent death like an instaheal would - heck, Pain, Merlot, and Furor give a self-heal, making them more versatile Tomes overall.