Are you sure that it doesnt stack?
The vulnerability for element fro monk and sorc? I never tried but i think they stack, need to try out.
They have seperate numbers, seperate debuff icons and dont override each other. Should stack no matter how i look at it.
I solo to much as sorc or duo and /buddy doesnt play melle monks/ so im not sure if it stacks or no.
A good split for example "what i used" was a druid monk wiz caster a long time ago and freezing spray stacked with word back then so i had both debuffs for extra cold damage, i just had to approach and hit enemy once to proc the debuff (was regular 17 druid 2 monk 1 wizz build) and the 10% extra helped a build that severly lacked boss dps back then /was when wgu was end game
I used th staffs quite regulary to proc the 20% more damage, you cannot ignore that debuff, you can stack it fast then swap to sireth or whatever and you wont use mortal fear if your staff is a debuffer staff anyways.
I generally play with many gear swaps, i dont like to put 1 weapon and call it a day.
Same as when i played a 2wf ass rogue for a while.
I had soooo many sets of weapons that my head hurt when i had to trash all those epic items
IF you dont do that you lose more dps then you would get without the debuff in boss battles.