I have always thought the primary reason the Marines want an aircraft is to protect their guys on the ground. I have seen several reports from Afghanistan and Iraq where the bad guys were just across the road from our guys. That tells me the only weapon to use is the gun. The A-10 works well in these situations because it is accurate with its gun. I cannot come up with any other air-to-ground weapons that would work in these situations.
Go ask any fighter pilot let them explain it to you, to a man or women they want one maybe two.
Engineers been trying to get rid of that gun for nearly 60 years now. Its usually elite aviators first question now when evaluating new designs.