I agree. With a good weapon PI's still go down without too much of a problem. And with a good merc they are even easier so if you can use the points elsewhere then you would probably be ok.With a good weapon, I find that even with no added points in Berserk, my barb can take out PIs.
And as I mentioned before, PIs are pretty rare compared to other monsters. If you find yourself doing "enough" damage, then I guess sacrificing some points into Berserk would be fine.