Air shear can generate a significant amount of heat. Reducing drag will also reduce the power requirements for the motor and voice coil. HGST claim a 23% reduction in operating power over air-filled drives.
As I understand it, the crucial factor motivating helium-filled drives is the reduction of vibration due to internal turbulence. Reducing turbulence inside the drives allows platters and heads to be made thinner and packed more closely together without affecting reliability.
The servo and associated electronics surely generate many orders of magintude more heat.