True, which certainly mitigates the level of downgrade. But a re-rollable 4+ wounds 75% of the time. Wounding on a 2+ wounds 83% of the time. That's why I said that toxin sacs are (going by RAW) a downgrade for a carnifex unless you're attacking T8 or better. A re-rollable 4+ is better than wounding on a 3+. It is not better than wounding on a 2+.
This is why Toxin Sacs on Hormagaunts are awesome and Toxin Sacs on Carnifexes (and, for the most part Hive Tyrants) are useless. As for those of you that have modeled Toxin Sacs on Carnifexes, I don't believe you are required to purchase it, but must have it modeled if you do purchase it.
now this makes the debate about tyranid close combat biomorphs interesting again!
why? because now there is an actual up side to seeing them as special weapons. namely that a hive tyrant/carnifex can choose if he uses his poisoned weapon or his other (bonesword, scyting talons) weapon.
reroll 4+ against other monsters and squish infantry still on 2+ ^^.
on the matter itself I think the codex writer did not know the fine parts of the poison rules. otherwise our mosnters would have poison 2+ (like the nurgle monstrous creatures!).
I'm wondering if GW alltogether know what they have written or if it was a mistake? cause only useing the poison value if you were worse otherwise and rerolling your normal s/t roll if your strenght is enough to reach the poison value would be more sane.
Except it doesn't as Toxin Sacs are not weapons of any sort. It's a "Biomorph Upgrade" i.e. wargear, thus would apply regardless of other weapons. So it doesn't matter how the Tyranid CC Weapon debate is ruled in the end, attacks are still poisoned.