Having ships do bombardment on their own creates a serious unbalancing issue. Either it is cost-effective or it is not. There is no real middle ground. Then you play a cat and mouse chasing random bombardment groups trying to kill your production in enemy harbors not bear any threats. Then the enemy has to constantly shift and move around units to protect against bombardment.
On page 83 of the manual it says: "Any of the attacker’s fleets not in a port next to a defender may also add in their naval bombardment support to the attack."
If this is accurate, where does this support show up in a report? It would seem that if the SAG does add support, then the defending unit should experience some level of damage or suppression.
I just replayed the Okinawa scenario and payed closer attention to the combat log. Sure enough the SAG did support the attack with bombardment. Don't know how damage was inflicted but at the very least, it did as the manual said.