Shane, It's not unethical at all. He hit you and caused the damage so he owes to have his damage fixed. If that also fixes old damage, it's just your good luck. But if damage is not in exactly the same place, the adjuster is good, and they can fix his damage without repairing the old, that is also fair. Insurance adjusters are normally smarter than they seem. Watch them like a hawk. Good luck.
Yeah, I'm trying to be fair about it- the tree did damage to the bumper und the trunk lid, but I told the guy who hit me that the trunk part was pre-existing- kinda omitted the damage to the bumper section in that conversation...