It's not that Sword Oath isn't good or useful. It's just that it's not at all interactive. You're gonna toggle it fairly rarely in a normal situation and the rest of the time it's just there. It doesn't give anything to the paladin. There's no compelling mechanic. It's just "you do slightly more damage than you'd otherwise do while not tanking".
It's especially painful in a game like 14 where abilities are a premium.
