Sadly, whilst I'd love for us to regularly get a CAS helo (with ATGM's, not just rockets), I'm inclined to agree with Persona. A rotary CAS aircraft would, for most AO's, be entirely overpowered and would wreak destruction and chaos on the enemy....until it ran out of ammo.
ARMA does some very silly things on a very regular basis (re: physics and ordinance), but terrain masking is terrain masking. If you do it properly and you do it often, a CAS gunship helicopter would be able to pop-up, acquire, pop-down and then lift again to launch. With Fire-and-Forget missiles, the helo's are extremely survivable.... just don't let the enemy AAA have the chance to lock you. ?
Regards,
SandStorm.au