Dark swarm mechanics is simple. You probably know what dark swarm does, but judging by my past experience on you- I'll try my best to explain. And try your best to follow.
The most basic ZvT basically goes like this.
Z takes nat, take a 3 hatch build route, go mutas, harass until first marine batch comes knocking at your doorstep.
Usually against any muta build, T prefers 1 rack double cc to 4 rack builds- +1 upgrade. you block the first batch after stalling some time with zlings, mutas, and sunks- you probably wouldnt be able to harass once marines get +1 upgrade, so by then you should have some lurks. when first batch is blocked, you go straight to getting 3rd- which is important when you're going hive, and THEN you get straight to defilers, and DS- which is a relatively mid play strategy. By this time, T should have one or two tanks and sci vessel with irrad coming up. note that one or two tanks does NOT mean the terran went tanks. Theres a difference between 2 fac and 1 fac 1 port build. Now here's the important part. By now you should have a group of lurkers, shit ton of zerglings, and maybe mutas that you haven't lost, yet. Now normally, T army WILL rip apart any defiler-less Z army at this time. But if you have defilers with consume, then T will have harder time, and it won't rip Z army apart when you're inside swarm. Note that you should have a few scourges out to chase sci vessels, or they will just irrad your defilers. Remember to micro them. You can continually cast dark swarm to follow them, they end up losing parts of their army. Go for a 4th immediately, and get ultra tech. etcetc and so on.
But the point is, what DS does is keeping your army alive. Without DS, full upged rines will rip apart about anything you throw at it, and you'd never have enough plagues to cope with all of them.
Even if zerglings are expendable, they aren't infinite.
And plague research takes time, longer than you know. late game plague has some use, but mid game? You'd be wasting your valuable resource, and maybe valuable timing that allowed you to go for a 4th. If you can't make a 4th- then you're probably screwed.
And uh, late game if T can collect a lot of vessels, like 10 to 12- then you are also f'ed too.
And the key part of zerg play lies within defilers, and if you can't cope with that- I'd suggest giving up playing Z. I can agree that microing defilers at first isn't a pretty sight, because it is slower than other units- it takes some hotkey micro.
I did get a tad incohesive, but I think you'd get what I'm trying to say- since you've been throwing units at T without swarms.
If they went tanks, you shouldnt be getting DS anyways. But you probably knew this.
You shouldn't be going lurks either.
Is this just a phase that all z players go through until they grasp defilers?
Z has the most steep learning course because of the fact that every move they make requires defilers in order to compete in firepower.
None.