The debuff already has a low proccing rate, when used against bosses, it’s even lower, possibly within the 10% area.
Even then, you’ll have a short of amount of time to take advantage of the debuff, skill animation taken into consideration, you’ve already got less than 6 seconds, not only that but with the supposedly new boss AI, you’re likely to be knocked back, meaning less time to take advantage of it.
One thing I don’t know is if the debuff stacks in both pierce vulnerability and time.
Personally and currently, the amount of negatives with Spear Lunge certainly outweighs the positives, by all means take level one for whatever reason, it’s simply logically not appealing.
If the debuff procd 100% no matter what, that positive alone would be enough to outweigh the negatives.