The hydralisk is probably the most cost efficent unit in the game. They have a great rate of fire, and can deal a great amount of damage to large units. Easily mass produced, they are one of the cheapest units in the game.
Hydralisks are necessary for anti-air defense, since mutalisks aren't really a match for wraiths and scouts. Hydralisks are also great units to run over a well defended base, because they deal so much damage to structures. They're also powerful units to hold a choke point, since they have decent amount of HP and a good range attack.
Hydralisks upgrades are important. The range upgrade can help to get more hydralisks involved in a battle. The speed upgrade is essential for those long treks, though not necessary. The missile attack, can greatly change the outcome of a battle, but the carapace might be more valuable. Since marines take 1/2 damage from a hydralisk, adding 1 more point of damge only to have ot halved is too good.
This is especially when marines do full damage to hydralisks. Same with hydralisks, they only do 3/4 damage to other hydralisks. Up to you to decide which is better, but in the long run, more units usually wins out.
