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 counter. Fungal for the blinks + anti armoured Ultras with splash. Why less ultras and more money invested into other units, not so hard counters as Ultras, works better???
Also, there are other questions regarding zerg play, that many people have: Inject larvae is absolutely necessary in the early/mid game, so that almost all zerg players feel obliged not to miss a single one of them, but as the game progresses they feel less and less compelled to do it, as if it suddenly becomes more forgiving. Some will argue it is because of more hatcheries. Valid point, but during the end game the needs are also a lot bigger, and you are in many cases needed to replenish a whole army at once. So why does this happen?
Trying to answer those and other questions related to zerg gameplay, I took notice that in my earlier replays, there were some games where I had extra supplies but not enough larvae to spend it on, and some games I had many larvae but not enough money to use on them. At the time, I wasn't using diverse armies, but I was sticking to a more basic army composition.
Watching the Fruit Dealer playing at the finals these days, I noticed he was building a few of many units to win: His army is composed by Slings,Blings,Infestors,Roaches,Mutas and Ultras as he does his final pushes in many of his matches when he reaches end game!!! He only seems to neglect Hydras, as it is usually subpar than his other choices, but still, its amazing. Why to spend more money to build more zerg buildings to open up the tech choices and more unit types, and also have to tech more upgrades, rather than build more army and roll them over??
Thinking all those questions I think it all comes to this: Larvae
Larvae is the unique Zerg resource, and the reason why most people find Zerg more complicated than the other races. It is indeed hard to manage your larvae well until you get the grasp of its fundamentals, which is the following: Unlike the other races, the zerg is PUNISHED in their production the more they build weaker units.
To explain: When a Terran builds a Marine, he uses 50 minerals, and also uses a production unit (queue in his barracks) for 25 seconds. If he builds a ghost, he uses more resources, 150minerals 150gas, but also he uses more production, as he will have his barracks unavailable for another unit for 40 seconds. The supplies used/time favor the ghost of course, but nevertheless the extra build time is used to balance it out. If a Terran chooses to go Marines only, he can do so a lot more easily, and we shouldn't forget Reactor. So, Marines might be cheaper, but also they come a lot quicker, so you can have more of them in the same time as you can have 1 ghost. A reactor considered, it is about 3marines=1 Ghost using the same time for the same production facility (Barracks). So, If you want to build a 2250 resource army of Marines (47 marines), it will "cost" you 587,5 seconds of barracks usage. If you build 2250 resource value of ghosts (6 ghosts), it will cost you 240 seconds. The Marines stress your production about 2,45 times more than the highest cost infantry.
Now let's consider zerg. If you want to build let's say a 2400 resource army, and this is consisted of pure lings, you need 48 larvae. If it is Ultralisks, you need 4!!!!! This is HUGE difference, 12 times the production cost!!! So, even though your army is equal in terms of cost, it is HUGELY uneven production-wise!!! This happens because Zerg production isn't affected by time!!! There is NO queue, and no matter how long your forces will do to spawn, the larvae will keep popping at the same time intervals. Inject Larvae doesn't change that, it boosts larvae production by a certain amount, sure, but still, the usage for an army of lings will be huge compared to one of Ultras. All this is nice and all, but how often does one need to make pure lings? I'd say almost never, but even if the enemy is only sending in Stalkers, in not critical mass (small groups), which lings are supposed to hard counter, a zerg player would have WAY better results if he builds other units as well, because if he doesn't, he is like sabotaging his own production. That way, he gets production blocked, which leads in a huge surplus of resources!!! Now if he chooses the other way, to build mass Ultras, which AGAIN are supposed to hard counter Stalkers, he will use only a little portion of his production capability but more resources, which will lead to a surplus of larvae but lack of resources!!!! The OPTIMAL choice would be to build Ultras AND Zerglings by that ratio so that he has 100% larvae AND resource usage!!! Or else he is letting a bit of his economy unexploited!!! (Make no mistake: Production is a VERY important economy factor). Of course, in this example, he would win anyway, BUT in actual matches, there are never so simpe cases. The fact stands: A zerg will NEED diverse army because he needs to ALWAYS even up production and resource usage and he CANT with an army consisted of 1/2 unit types!!!
THAT is the reason why the early game inject larvae is ABSOLUTELY necessary. The 2 most production-stressful zerg are dominating the early game, that is the zergling and the DRONE. Yes, the drone is actually devastating to zerg production, because it is UNAVOIDABLE to make lots of them early, or you lose the match!! They are in the exact same spot as lings, 50 resources for a larvae. If you add many zerglings to that, you have a REALLY hard time for your production, thus in the early game you need to be injecting like crazy or fall behind!! Solutions: Fast Expand, (wonder why it feels so absolutely necessary to expand as zerg?? One reason is this, there others of course too), OR, add Roaches to the early game equation, and a few Spine Crawlers! As the game passes, you have more options, but REMEMBER, you have to use a nice combination of cheap/expensive units!! No, it isn't a choice. You HAVE to.
Wonder why mutaling is so vastly superior to Hydra/Roach? Many will say because Hydras and Roaches suck. Sure, they can use a buff, BUT mutaling kinda FORCES good larvae management to you. Mutalisks are production light and zerglings production heavy and they end up in a nice balance. But it comes rather intuitively, because Mutas also gas-starve you at the same time, so you HAVE to build lings to use the excess minerals. Other builds aren't so simple. Hydras and Roaches are both similar at the Mineral/Gas ratio, and much closer as far as production usage is concerned. A player can't easily understand why making many roaches and a few hydras will serve him better than mass roaches against marines i.e. It isn't intuitive. This build, actually, to be more effective, needs a few lings added if many hydras are needed as hard counters to the enemy (or more drone production), or another more expensive unit if roaches are more needed (many choices here). If you choose a roach heavy roach/hydra then, you are FORCED to tech up asap and build other units or again, your economy usage will soon cripple your play.
Anyway, this kind of weird production principle of the Zerg is an artificial method of forcing synergy and diversity to the Zerg BUT it isn't easily seen as with the other races... People don't understand it and for them Zerg seems dull.