Dark Swarm...What does it do exactly?

Spawning pool, lair, queen's nest, and hive are requirement buildings necessary to advance the tech tree beyond the hatchery. If you picture the tech tree as an actual tree, the hatchery would be the roots, pool/lair/nest/hive would be the main trunk, while the rest of the tech paths/options would be the branches.

Building a queen's nest to upgrade to hive and get to tier 3 is very different from actually riding queen tech. Just like having a lair, or even hive, doesn't necessarily mean your overlords are unit transports.

Although it is true that you can still produce queens because you've built a nest, making queens that can only cast parasite, having no way to directly influence battles whatsoever, isn't very useful. Even fully researched, the queen's usefulness is "debatable" at best. But just to get all the researchable upgrades, you're putting down 350 minerals and 350 gas(ensnare 100/100, broodling 100/100, +50 energy 150/150). Then you have to consider that each queen is 100 minerals and 100 gas. All these resources dumped into queen tech are resources that you could've put into getting defiler tech sooner, or at least getting more combat units while you tech up.

To put it bluntly, the queen just sucks. That's why better Zerg players never waste on queen tech.

 

Here's to a tier 2 Zerg spellcaster in SC2!! *clink*

I still vote that the Queen is worthless. You have to tech her fast to have it be useful. Only good for slowing down units and the occasional broodling of a HT.

 

The Queen is not as useful in "competitive" games, ladder, etc. because such games are played on smaller, open maps. When you're playing on a map where mobility is highly limited, or there is just too much terrain to cover, the Queen's Parasite is incredibly useful. Not only is it placed with a flying unit, Parasite will let you scout without a unit, without alarming your enemy. If they do catch the Parasited unit, it is as if the unit has been Broodling-d-- unless they have Restoration, the enemy has to send the unit somewhere useless, kill it, or send it off on a suicide mission to keep the player from spying on them.

Again, none of this really matters in the more cramped spaces of common play, but there are real games where it counts.