Dont listen them. Necro3 is PvP only, and only vs very serious gear and with psy2 at least. Necro3 disables atk,def and mdef from equipment. Skeletons are meat shield only, the deal almost no damage.
Necro1 can tripple your dps from Temple Shooter when we get fix from ktos. Necro can put 2 different decay 1 from Shoggoth, 1 from FC or DP(this 2 are same) and they stack with each other, so you get +200% damage for Temple Shooter. For PvE go Cryo3-Sorc2-Necro1-Sage1, this will be pretty good when we receive the buffs. Sorc3 is useless for now.
So, I would say for PvE necro1 is enough, unless they improve this class like they did it with Sorc2(TS is now missile attack and gets up to 2x your matk as damage), there is no point going necro2 and 3 beside for afk farming.