Does anyone have emperical information about the mechanics of Insect Swarm and the new Scorpid Sting? Specifically, are these debuffs incorporated into a one-roll attack system such that they are additive to tank avoidance, or are they incorporated as part of a two-or-more-roll system such that they are multiplicative? After the change to Scorpid Sting went live there were several anecdotal posts that tanks were not noticing significant changes in mob miss rates and that the duration of Scorpid Sting was just too short to make it worthwhile. Similarly, there have been some posts describing the chance to hit reduction of Insect Swarm as being practically worthless. However, I have not seen any hard evidence either way, and I am curious if a stacking mechanic makes these abilities more useful than they initially seem to be.
It's an interesting question. For Insect Swarm I had a brief conversation with my MT about using it on Patchwerk - his high hit rate could have potentially made it a very worthwhile debuff but with the healing requirements of the fight and limited numbers of healers I simply didn't have the time/mana to cast IS on top of FF and still keep my tank alive during the fight.
I'd imagine this would be pretty difficult to test. Probably the easiest way to test this would be to use a rogue. Mobs have a default 5% miss rate against even levels. If a rogue has 38% dodge/parry, and then pops evasion to bring that up to 88%, they'd have a 7% chance to be hit. (5% miss, 7% hit, 88% dodge/parry) If you then put insect swarm and scorpid sting on the enemy, they should have a 0% chance to get hit if they both stack and it's a one roll system (88% dodge/parry, 12% miss). If they still get hit then they either don't stack, or it's some sort of two roll system.
38% dodge/parry isn't out of reach for a rogue if they really go all out on it, is it?
That priority system makes complete sense, but even though it implies a one-role condition, I think a test is worthwhile. Perhaps this has been resolved elsewhere during investigation of combat mechanics, but I can see two possibilities for how these debuffs are applied. One option would be for the misses to go into a single attack table along with dodge, parry, block, crit, crush, and hit. If that were the case, then the chance to miss would effectively be additive with dodge and parry, and would be quite good for mitigation. However, its also conceivable, if unlikely, that the mob could first roll a self-miss check and then roll against the player's avoidance. In such a two-roll system, the benefit of Scorpid Sting and Insect Swarm would be much lower. Of course alternately, Scorpid Sting could just be bugged. I'll try to arrange some testing with a rogue later this week, unless someone else has some test results they can post.
There is a threat here somewhere where someone tested it (ages ago, so it could have changed by now) and turns out it does stack, but it won't lower the mob's damage by more than 30%, if I remember correctly.