The comment about this change just resulting in an increase in mob damage is likely correct, but that increase in damage will likely be relative to the benefit to robe users not to Heavy armor users. A robe user with maxed PRR/MRR will take 150/250 = 60% damage from all incoming magic which means we could expect base damage on spells to go up to 100/60 = 166.66%, a 66.66% increase (since ALL characters can be expected to be able to reduce all magic damage to current levels without much difficulty). With this increase in mind what we are really looking at in terms of numbers is:
Robes (and no heavy/tower shield): Same magic damage as now
Light Armor (and NO heavy/tower shield): No save = 150/350 = 42.86% * 1.66 (expected damage multiplier) = 71.43% damage -- effectively 28.57% reduction
Medium Armor/Heavy Armor (WITH heavy/tower shield), Lets go crazy and assume that this character somehow managed to achieve 300 base MRR:
Vs. effects with no reflex save character would take 150/450 = 33.33% * 1.66 (expected damage multiplier) = 55.56% damage -- a 44.44% reduction
Vs. effects which allow reflex save character would take 150/750 = 20% damage * 1.66 (expected damage multiplier) = 33.33% damage -- a 66.67% reduction (83.33% reduction with perfect reflex save on a tank)
A few reminders to keep in mind:
Characters with perfect evasion (no fail on a 1 and high reflex save) will always take no damage from sources which allow reflex saves (which represent the majority of offensive spells) -- this is not comparable to the damage reduction on a tank even with perfect reflex.
Characters with improved evasion, even with an AWFUL reflex save take 50% damage from all spells which allow reflex saves -- a 50% reduction
so: Characters with dominant reflex saves and evasion are still the best in the game BY FAR at handling AOE effects -- tanks just won't get melted anymore.
Improved evasion will still be a powerful and sought after effect
Heavy Armor (and medium armor) will be much more sought after than they are now (much needed)
I would say that the problem with this system really boils down to the removal of the "ranged touch" role from ray spells like disintegrate, searing light, scorching ray, polar ray, etc. This armor update makes tanks unfairly powerful versus spells which allow NO reflex save -- the change versus effects which allow reflex saves is very well done. In traditional pen and paper, high dexterity, or high dodge characters are commonly missed by ranged touch spells like disintegrate (so even though they can't evade some/all of the damage, they can dodge the incoming ray in the form of the caster failing to hit on their "ranged touch" attack which gives them the ability to mitigate incoming damage) add this to the calculations for ranged touch attacks and I feel it would perfectly balance the system as intended.