I don't tackle the spiders until I'm lvl 5+ and geared up, and the SQ has still killed me more than once without actually poisoning me. Just miss a couple of attacks and/or do low damage when you hit.Daiyu's shop sells Healing Potions and Buff Potions from the start of the game. If Cure Poison could be bought too, that would make entering the Spider Cave for the first time a fetching mission instead of a Main Story mission.
The Spider Queen is one of the first Boss Figths. Poison immunity make killing it easier than killing a Wolf or a Bandit.
Before Daiyu's shop was added to the game we already could easily stock up on Bandages (100% heal for 40 coins) and buy decent Gear if you take the time to do a few missions around Weirdwood.
It would make sense to be able to buy Cure Poison after Zivah breaks her curse.
Getting the spider eggs isn't a quest by itself, just a step for breaking the curse. IIRC you need to break it before Makith sells spider eggs anyway (she doesn't sell swamp toads or blood grass until after going to the Swamp, either). We already get a Phallus Giganticus potion for Maghda, why not a Cure Poison? Use it wisely! At worst, add it to a Cheat chest.
Experienced players like us won't need one (though some want it anyway), but we're discussing possible ways to make it easier for newer players and/or logical things to add.