As Yukichibi points out, Cover is a fundamentally flawed JA. You "Cover" someone who has more Enmity than you and take hits for them, which reduces your Enmity further and makes it harder to gain more Enmity than the person you're covering. It's a stopgap maneuver that you use when you hope the fight will end soon and you have no way to actually take hate. Making it a job trait would just rearrange party members so that they're all hiding behind the Paladin. It would make Paladin "useful" in that normal DDs wouldn't die to regular attacks, but no one dies to regular attacks at the moment anyway.
What would actually "fix" Cover would be redirecting enmity lost from taking damage during the JA to the players behind you and giving the Paladin some kind of CE bonus for damage mitigated during this time. Then you'd use Cover and come out of it with more Enmity than you entered it with.
It's difficult to justify bringing a Paladin anywhere at the moment because nothing is damaging enough that a DD couldn't tank it. Making Paladin able to hold hate 100% of the time wouldn't really change this. The true "fix" to Paladin is just adding hard monsters to the game outside Abyssea.