It is 10 points cheaper then agoniser, so you can give those en masse. I cant justify giving agoniser to 5 man wych unit, but can giving them power spear.
Also it is affected by drugs and Furious Charge.
Also on Hellions it is essentially better agoniser, with their hit'n'run.
We make most of the damage during charge anyway. Rightly so now with worse Feel No Pain.
Tis also great on HQs who can take 2 close combat weapons. Cheap Archon can take Power Spear, Venom Blade Combat Drugs and it is very viable load-out.