Well the AAC/idle control valve controls engine speed (by a an amount) with a PWM modulated inlet leak.
That small amount of adjustment is required to give you a stable idle even though there is load appearing and dissapearing.
A/C, h.beams, stereo etc etc, as well as catching the motors decel after depressing the clutch.
AFAIC, if you don't use one, your idle will fluctuate with engine load at idle. That idle RPM in question will need to be set quiet high too, to prevent stalling when above said "motor decel, clutch press" occurs.
Interestingly, i guess a race car wouldn't need one. Could you let us know, why your thinking you don't need one?