I wouldn't rely on priests alone because:
- When you go out of mana, you die. There is no melee alternative and armor like shamans have. You could use Psychic Scream 5 times in combination with wands, but it's not going to be effective enough when attacking multiple characters.
- How are you going to do instances on your own? Just using Psychic Scream won't work, especially not on bosses.
- When your whole team wipes, you don't have wipe recovery.