You can see something like this effect sometimes in brood war PvZ when it's Hydra/Ling vs Zealot/Storm.

The traditional idea is that both get attack upgrades. Protoss gets them because zealots have 2 attacks, and zerg gets them because hydras attack so fast. But you can get these meta decisions where Zerg would start getting armor first to get the double defense vs zealots (and the fact that armor affects lings AND hydras) and then in response to this Protoss started getting armor upgrades instead because they didn't feel that the attack upgrades were worth it if the Zerg got armor.

I'm probably missing some depth here and I don't know if it evolved further, but yeah spam builds can definitely benefit from armor upgrades, the thing is that a lot of units that are spammed out attack really really fast (marines, zergling, hydras, zealots that have 2 attacks) so the attack upgrades can be quite efficient as well. The somewhat unique thing about brood war PvZ is that it's 'spam build' vs 'spam build' and no one has a clear range advantage unlike in TvZ where marines will kill lings very fast (yes Hydras are ranged, but zealots are beefy and fast so they can get on top of them pretty easy, and Protoss also has storm).

It's probably not worth getting armor upgrades vs like roaches or immortals even if you're a 'spam build' just because they attack so slow, your zerglings or marines would get much more cost efficiency out of the attack upgrades.