The 8 AP to launch is an good counterbalance.
An attacker can jump on just prior to midnight with 2 ground wins, and within 24 hours can end the war and beige you, taking a percentage of your loot and your alliance bank's loot.
Might he take a lot of infra dmg in the process? Yes, if target have missiles, target might launch 3 against attacker (if active, the attacker is always active, obviously) in the time it takes attacker to beige him and get the loot. That doesn't seem entirely unbalanced to me.
Defending with just missiles is a risky move. A gangbang will be devastating as you'll run out of missiles rather quickly.