That is the fundamental design. They are strong against a few marines once they have speed because they can get a surround, but weak against a critical mass o

That is the fundamental design. They are strong against a few marines once they have speed because they can get a surround, but weak against a critical mass of marines, since the surface area to damage ratio of the marines decreases. Also their speed upgrade gives them mobility to counter attack exposed buildings. Also their half supply means that they can be worked into compositions when close to the supply cap. Also their fast and parallel time to make means they can reinforce fast after a long battle. Also their general upgrades and late game attack speed upgrade means they can take down buildings fast if there is nothing around to do damage to them. Also if they are spread out they can draw splash damage from power but slow firing units like siege tanks. They can also be sacrificed to burrow and delay an enemy base, used to scout cheaply ... All that is just from the first attack unit in the game.