Yes if you face high resist persons. However I haven't seen any yet so it depends on circumstances. It really helped me against some opponent in 2.5 when there was an opponent with really high resists.
It's as good as 20% AR piercing in 20% weaken crystal affix.
It is subjective though. Ascendant is alright for PvP if you have the correct build for it and goes well with weaken. Larrybird is one good example in my facing against him.
I believe since cap of weaken is 200% , PvP cap maybe 40%?
So basically, kind of?