There are several useful professions for priests.
Engineering
Engineering provides many useful doo-dads that can help in battle. Grenades, while large in dealing damage, more powerful grenades stun opponents and can give an advantage in combat. Engineering also provides a net gun (for snaring rogues), and many other useful tools.
Alchemy
Alchemy provides those essential mana and health pots that a priest can never seem to get enough of. In addition, alchemy provides the best method of earning income of any of the professions.
Enchantments