Gearing is of course not as simple, you sometimes may wanna get more spelldamage/+skills rather than sacrificing everything to hit that capped pierce. You need to decide when pierce beats other stats, it isn't set in stone you HAVE TO get -130.
They still deal enough damage even without fully minmaxed gear. Also source of pierce isn't always crafted item.
I did try fire a3 merc on my last barb, because I used to have nice fire caster gear and jewels to use, but mechanically fire spells such as pyroblast just are bit clunky/slow for good clear, damage is there but the delay/aoe pattern seems worse than lightning merc which primarily uses forked lightning.