Also a bit more on the pdef/hp issue coming from a healer here is the reason for it.
For every other class mainly dps you here to avoid defense stats like the plague and grab hp instead. This is because those classes are attempting to push their hp up high enough to breach over a flat rate of damage with a little breathing room, but with a Knight extra hp just pushes your hp even higher over that amount but the attacks from the boss will do more damage then if you had been stacking defense instead.
Now as for where healing comes into this... lets say that you stack either hp or defense with equal effectiveness. The person that stacked hp will take more damage from hits and so they will still take the same number of hits as the person that stacked defense. So lets look at two scenarios
Tank#1(HP) Tank#2
100,000 hp (100%) 80,000 hp (100%)
-60%hit(60,000) -60%hit (48,000)
40,000 hp (40%) 32,000(40%)
+50k heal +50k heal
90,000 hp (90%) 80,000 hp (100%)
Granted this is a EXTREMELY simplified example you can see that while both tank 1 and tank 2 take the same amount of damage as a ratio to their max hp the higher hits of the first tank would cause him to be unable to take that boss while the second tank would have no troubles at all even if a bit of extra damage was through in here or there.
Also a thing I see many knights do is they almost completely ignore the importance of their attack stat. Unlike most games the Runes of Magic threat system is directly tied to the dps and aggro skills work by adding a threat modifier to the amount of damage done instead of creating a flat taunt. So if you run into an isntance with 50 million hp and 300million phsical defense but only 15,000 physical attack you wont be able to do any damage so the scale multiplier will be scaling a very small number and you wont hold aggro. At that point the highest dps in the party becomes the one taking all of the hits and your super geared knight ends up turning into a rather fail dps class.
P.S. I wanted to touch on something rem said early on about knights being the worst tanks... while that is theoretically true the issue comes to costs. The main gain that Wd/w and W/k have over the knight classes is that they have the potential for far superior damage output which means faster boss kills. Now it would seem kind of silly to have the best dps tanks being the best tanks overall so the reason for that is if you can survive the instance regardless and hold aggro regardless it comes down to what ELSE you provide mainly being contributing to the dps of the party. This is often why you will see skilled knights switch to non aggro but more damaging skills after the fight has started rolling along. Now where knights come in is that their skills are far more oriented towards their survivability so they have the capability to run an instance with less gear then it would take a Wd/w or a W/k to run the same instance. And this can sometimes mean that a W/k or Wd/w would have to wait till a knight ran the instance to get the new gear and buy down as they may actually need the higher stats of the gear inside the instance to even be able to run said instance. So while on paper Wd/w and W/k beats out knights the actual costs is unreasonable for most people.
