4 zerglings cost 100 minerals. 1 zealot costs 100 minerals. 4 zerglings kill a zealot. This is why zerg will ALLWAYS make 10-12 zerglings in the beginning of every ZvP and ZvZ they play. Zerglings + mutas is the only way to beat a good zerg player. Zerglings + lurker rush can waste terran. Zerglings own zealots in early game, they own goons in mid game, they own tanks (tanks kill lings??? tanks cant even attack lings when they get close!), and if u get many of them fast enough they even own marines.
And adrenal glands is not the only good aspect. Adrenal glands will only be researched in ZvZ, but zerglings are useless without ling speed.