The undersuit is the weakest point in a Spartans armor only being able to deflect 3 50 cal sniper shots before it weakens. The undersuit is mostly for Spartan comfort and temperature control with slight protection. Most protection comes from a Spartans over shields which buck couldn't have had since he had no helmet when fighting Mickey. The suit needs the helmet to power the shields so that the user doesn't get injured and because the visor helps power the shielding.