Ranged attacks. Especially ones that can potentially one-shot you or deal 60~80% of your health.
Mass heal level 10 is 30% health of all players in the party and the flat bonus is decent. You still have 5 x heal tiles. Hanaming gem will indeed add +1 to your heal tiles if needed.
You are underestimating Priest c3 > Paladin c3 apparently. 
As a Priest/Paladin you are a full support. You are providing enough support that missing out on Plague Doctor doesn’t actually matter.
Healing Factor only works on one person and has a fixed duration in PvP that is not increased with skill level. Bloodletting is no where near 100% uptime anymore, leaving you and your whole party vulnerable to CC.
Barrier magic defense (SPR) is relevent to a Priest c3 because you get SPR for cc-resist, mass heal, stone skin, aspergillum (chaplain).
Multihit damage is greatly reduced, so your party won’t get shreaded by pole of agony or icewall + pp combo.
Barrier magic defense is effective against % scaling skills like Meteor.
Resist Elements also helps in that regard, for elemental damage. Also has 19% chance to nullify elemental attacks.
Of course I advise always that they should choose that option if they want to.
However those who like Priest + Paladin skill have a viable class path, and can choose c3 paladin at rank 8 if they intend to play a full support Chaplain.
There is plenty of synergy with Priest c3 and Paladin c3 and was a meta on kTOS. For a good reason man.
Paladin is the much more defensive option than Plague Doctor or Oracle however it’s not the greatest at DPS. How you choose to stat your character doesn’t matter so much. As long as it has synergy with other class ranks and fulfills a certain role by choosing c3 Paladin.
You get 1 phys.attack and magic.attack per level, so enemy defense is actually not as high as you might think. Punching past the defense is already achieved by leveling.
I’ve explained the DEX formula for Cleric in the FAQ since I updated it. You need to have other sources of DEX and Crit Rate for the initial investment to be worth it. Archer gets Swift Step, Priest gets Monstrance, for example. So DEX actually has much better scaling with certain skills.
100 crit rate is needed to even have a chance to crit at high level, that’s with enemies that have 100 crit resistence minimum. an additional 100 crit rate is only +14% chance to crit at level 280, once higher levels are added that scaling will drop off further and you’ll become squishier against magic with a higher investment of DEX.
But Cleric is not very tanky and melee builds will suffer from low CON. All of these factors taken into account, you won’t benefit that much by spreading your Stats too thin into DEX. You are better off with higher CON for PvP and PvE so you can survive longer. It is the best scaling stat.
Archer benefits more from DEX because of Kneeling shot as it also increases your attack and running shot because it amplifies it further.
Also, why try to compete with Archer in DPS when you can focus on keeping buffs active and people alive.