Priest spells are wonderful for buffing and debuffing. Just at first level you have spells to increase the armor of your group, decrease the to hit of the enemies, a significant single target debuff for minor bosses.
At second level the heal over time area of effect spell is crazy good. Park Durance behind your tanks and watch them stay up and stay up, or use it in the middle of a clustered group and heal everyone.
In general terms priests aren't just healers, they are buffers and debuffers, and good ones. They are force multipliers - drop the attack speed, accuracy, saving throws, deflection of your enemies, while increasing all of that for your own team and watch the rest of the crew clean up.
Yes, other classes can buff and debuff, but Durance (or a PC priest) is by no means useless.
And yeah, give him an Arquebus and the approriate talent, and he does reasonable damage.
Unless your character has high mechanics (unlikely unless you're a rogue), he's also the best lockpicker/searcher amongst the companions, and I put virtually all his points into mechanics other than level 3 stealth and level 4 athletics.