OK, ICMR gives morale per minute right. In addition to fluffing up your health a bit, rank 16 justice heals you for 120 health per minute. This is not modified by incoming healing.
Looking at innocence for an example of a mit vitue: Innocence gives 864 physical mitigation and some tact mit and resist too. Just doing a quick-and-dirty look at how much 800 something phys mit buffs my LM for, I took off an item that gives me a total of 837 physical mitigation. With the item, his phys mit is 34.4% and without it's 32.3%
So a decrease in my physical mitigation from 34.4%->32.3% means I take 3.2% more common damage. Yes 3.2% more damage and not 2.1%, because (1-.323)/(1-.344)=1.032.
How much common damage does it take to make putting on 837 more phys mit better than 120 ICMR for my LM in his current build? If Ulxart received 3750 common damage in a minute with his phys mit at 34.4%, he'd take like 120 more damage per minute if he had 32.3% phys mit instead. Any more common damage and phys mit saves you more health than 120 ICMR. That's a really low threshold of damage, man, even for a non-tank. It's a threshold specific to my LM but you know other folk's are gonna be in the ballpark. Having defined that threshold, I can now make the qualitative argument that the threshold where ICMR heals more damage than that bit of mit prevents is for damage so trivial that it doesn't even matter.
Concluding, the popular Innocence 16 gives 864 phys mit, 216 tact mit, and some resistance as lagniappe. In circumstances where you're actually taking common damage like 3man/6man trashpulls, against Durchest and the Black (as opposed to Shadow/Lightning/Fire) Armours in BG, etc., yes that will most likely prevent more damage than a piddly 120 ICMR will heal.
Now I think that answers your question about mit vs ICMR. Now, Justice has max morale too which can add a whole 'nother layer to the comparison, one that I feel requires breaking down by situation. Max morale + ICPR vs Mits against "sustained" damage and also against "burst" damage. It's someone else's turn to analyze though.
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