Have to take 2 units from CSM, BLs, or Zerkers for the slaughtercult so thats why i am fiddling with all three.
1) save points and take CSM or Bloodletters. Zerkers are glorified marines.
2) If you take 8 zerkers, you are bringing 20 cultists. Even with greater resilience and armour, cultists last longer. admittedly, leadership is an issue with them, but when you face elite army such as necrons, you want horde. if both units get to melee unschated, 8 zers will pump out 4 attacks each = 33 (+1 for the champ) for an average of 7 wounds inflicted to the usual necron warrios, while the cultists will pump out 81 attacks for an average of 6 wounds. Shure, lower than 1, but here's the catch. When charge bonus ends, zerkers will just make 2 attacks each; with 8 bodies...they are 16 attacks. Cultists will still pump out 40 attacks. And here is where they are better, they have enogugh bodies to keep pushing. Zerkers win first round of melee or just die of attrition. Each zerker is a serious loss in your attack value. Each cultists is just a chink in your dice pile.
Previous calculations are not considering the death of any of your models, but remember that after enemy retaliation, each dead zerker is worth 1/8 of your group, while each cultist impact only 1/20th of the total. They might die easier, but they count 1/3 of a zerker ;)
This of course is valid if you pit them against low-tier infantry. Vs terminators or say, spawns or Wraiths, you are not really using Zerkers, right??