except versus zerglings having equal attack to the Zergs armour isn’t that influential in the game.
The higher tech end doesn’t get as much use out of the extra attack upgrade (Immortals, archons, storm)
Zealots will still kill Lings at the same speed at 3-3-0 vs 2-3-1 but have a higher survival rate
Zealots actually kill the hydra with one more hit remaining with 2-3-1
and the extra shielding effects your buildings/probes (for those pesky run bys) , the prominence of shield batteries, archons not having a lot of value from plus 3 attack and giving you a leg up on your air transition by having one shared upgraded already purchased gives you considerable consideration on skipping +3 for that extra shielding piece of upgrade maybe worth it.
Every engagement survived gives you increasing value.
There are some drawbacks. Zealots will do noticeably worse against 3-3 roaches and also kill lurkers slower.
You miss out on the small +3 timing window you have where zealots will kill lings at that 33% faster rate.
and sustained fights with large zealot numbers can go worse because charge keeps the hits at a relatively same level but one it’s lost they often have to do another strike.
So what’s the verdict?
If you are going turtle you might find the 2-3-1 strategy far more effective giving you that extra value edge to snowball out of control. Especially with shield batteries and air tech switch.
If you plan on staying a primarily ground based comp and are committing a lot into zealots for aggressive posturing then the extra attack upgrades to kill buildings faster, doing better against roaches and leveraging that cool temporary 33% kill speed bonus against lings with zealots might be more your thing.
Edit: Shields are always trash versus Terran because of ghost and widow mine.
The high health base/High damage base of Protoss units lowers the value of shielding but I have seen +1 shields be chosen over +1 armour in Gsl recently for PvP in high archon based play.