I'd take him. Give him a year or two behind Palmer to heal up, learn the system, and he'd be worth it. It's not like he doesn't have talent.
A young QB groomed to be QBOF is still needed. You don't turn down a RG because you have Logan Thomas, or even Drew Stanton for that matter.The Cards already have someone behind Palmer and he's doing just fine. QB depth is not a need on this team.
He was just running when he dislocated his ankle. He reinjured his knee without being touched his rookie season. It's no longer a case of whether or not he can take NFL level contact.
Best case for RGIII would be to be traded to a small market team where he can get himself right physically and more importantly mentally. The Cardinals would be a good fit, but not at his current salary and not with him thinking he is a superstar. He is a reclamation project now and needs to understand that.