Not at all. What you want to do as you move forward is have sufficient threat saturation. Make it so that yeah, your opponent can kill a ridiculously expensive model with one shot, but for every shot they put there, they're not shooting at X, Y and Z who will also wreck their day. You want them to panic and try to shoot everything because it's all scary. Threat saturation at 1k isn't really easy, which is why I'd drop the Helcult for some scary stuff and get started on that idea early. Maulerfiends, Forgefiends and old fashioned tanks all make for great Distraction Carnifexes.
It depends what you want a unit to do. Units that aren't very killy can still be used as distractions or to leverage sheer number of dice. I like to run the Helcult against people dropping massed short-range firepower. Those unbreakable cultists are great at locking those shooting units in combat and sure, you're not going to expect them to win, but if they're not going to break after losing combat, if you have sufficient mass of them, you've taken that shooty unit out of the game unless your opponent does something about it. Wraithguard are a classic example, they can't point all those instant-death D-Weapons at your obliterators if they're stuck in melee all game crushing maybe 4-6 cultists per turn. If you're fielding cultists in squads of 20+ for only around 100pts, that's still a net-gain in points worth of firepower you've denied your opponent. The tarpit mob is an age old, tried and tested 40k tactic.
Just because a unit is not screaming "KILL! MAIM! BURN!" at your opponent every turn does not mean it doesn't have a purpose and just because an enemy unit isn't dead does not mean it hasn't been neutralised.