Misery and spell interactions like Catharsis are at odds with one another… friction.
One does not benefit the other.
Casting Vampiric Touch on a target while having Misery means you will never benefit from Catharsis type of spell interaction because you would need to manually cast SW:P on the target that already got your SW:P from your Misery Vampiric Touch application.
This is the friction.
So let me understand this…
We have to manually cast Vampiric Touch right? Doing so applies Shadow Word: Pain to the target (with Misery) so why does the duration of SW:P play a factor at all? Because you STILL need to cast VT when it’s duration runs out regardless if you have Misery or not.
So no matter what, we have to maintain Vampiric Touch on the targets which means in having Misery, you don’t ever manage SW:P… or you shouldn’t have too.
Yes, you can cast SW:P on the move, I get that. But now you just desynced your Vampiric Touch timer as you will still need to manually cast VT.
So instead of spending a GCD casting SW:P and then casting VT later anyway which will refresh SW:P regardless (with Misery) then why not cast SW:D or Instant cast Mind Spike or Void Bolt etc.
You save a GCD by casting VT and it applies SW:P with Misery, freeing you up to use another VT on another target or using a different instant cast spell on the move and you don’t have to think its a waste because you will just override that instant SW:P soon anyway when you refresh VT.
For me, I just use SW:D for that honestly if I really want or need to tap / tag a mob or totem killing.
I’ve gotten used to play that way because I learned how to prior to them adding initial damage to Shadow Word: Pain. I have adapted to not rely on SW:P instant damage as you have said before, you have better spells to use your GCD’s on.
There is good reason Misery was not only carried over into the new talent trees, but was made an early talent and forced. Because the player base in mass took that talent over other options on the old talent tree. Because again, for a long time SW:P did no initial instant damage. So it saved a GCD because we STILL have to MANUALLY cast Vampiric Touch.
So it made sense to have a talent that makes our always needing to cast VT to just apply SW:P and now you don’t have odd ball situations where you missed a target with a VT or SW:P and if you apply the missing dot, now things are desynced and it’s a mess if the fight persists too long unless you let them drop off and start over again.
With Misery, it fixes any alignment issues and ensures both dots are always up. Meaning we can get the interactions for one or the other spells without fear of missing that interaction because of a lag moment or something else that caused you to not apply both dots on same target multiplied by how ever many targets.