A 0/0 zealot will take a spine crawler from 300 to 180 health before dying. (taking range of spine out of the equation - so, assume they start attacking each other at exact same time)
A 3/3 zealot in the same scenario will take the spine from 300 to 102 health.
In particular I'm thinking about how upgrades scale against static defenses and how difficult it is for Zerg to secure their expansions lategame. For example, messing around in the unit tester, eight 3/3 chargelots can take out six spine crawlers. Zerg has to invest lots of money and drones to be able to hold against any reasonably sized zealot runby.
600 for spines + 300 for drones to make them > 800 for zealots
Are you seriously saying that static defense should be able to go toe to toe with an approximately equal cost of fully upgraded units? 8 zealots take up 16 supply! Assuming 60 probes, that's more than 10% of the protoss's entire army. Maybe zerg should peel off 10% of his army to go deal with it (or keep proxy pylons from going up in the first place).
isn't the point of static d super late game at your outlying bases not to completely shut down harassment, but to deflect it long enough for your actual army to get over there and defend? would be pretty OP if you could relatively cost-effectively shut down all protoss harass for 0 supply (negative supply, if you consider that many zergs overdrone just to be able to make a bunch of spines/spores later on).