I was giving a lot of thought into it lately, and I couldn't find a satisfying answer as to why the Zerg have to choose well-balanced armies and a variety of units to beat even their "hard counters". (examples: It is way better to go Roaches/Hydras than mass Hydras against mass Stalkers, even though Roaches aren't the optimal counter. Also, The popular build of Slings/Blings/Mutas works a LOT better than pure Slings/Blings even if your enemy is going mass marines and marines alone, which doesn't make sense. You'll still win with banelings alone, but with mutas it seems to work even better). Why does this happen? Zerg units don't have forced synergies, like MMM (anti air/light + anti armor/building + heals). And many will say that in some builds Roaches and Lings are used as meatwalls, but why the heck does Infestor/Lings/Hydras/Roaches/Ultras work soooo much better against Mass Stalkers than lets say pure Infestor/Ultras?? On paper, it is the optimal count