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.

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.