Actually I can get a Defiler faster than most people can get a Dropship. And what does expansions matter? If Terran got all expo's that wouldn't mean anything really. Just means that he has more SCVs working then he can handle and that means that his max will be low.
I play on non-money maps, i cant afford defilers in early game because if im playing against terran i will have to get hydras and lurkers which cost a lot of gas. I will have to make an expansion before any other building to get enough gas to pay for lurkers. In late game i will have 5-6 expansions so i can afford to make units, attack, and than make more units constantly. I will need 3-4 expansions just to afford guardians vs terran. Defilers cost gas, and to make dark swarm effective i need ultralisks + zerglings, and ultras cost a lot of gas.
And i never get more SCVs than i can handle. Expansions only have about 15000 minerals, so they run out and then i transfer the SCVs to a new expansions. After an hour me and my opponent will have spent nearly all the minerals on the map.
I will never reach my food limit until late game, because keep attacking and making new units constantly so i lose units. I have at most 60-70 supply of SCVs, and i dont need more then 140 free supply because i dont mass BCs or carriers.
Dark swarm and parasite are the only purpas for defilers.
Actually, queens cast parasite, not defilers. And plague is a very good spell.