[Ronin]

Damian's usefulness is limited by his inability to destroy figures without getting booted to the Start Zone when he's working with Batman (or another less-thematic Vigilante). Also Reckless Engagement + Bat-Grapple 3 means a lot of Jason's attacks can be attacks of 5, with some smart management. And occasionally getting to take a turn with him before your Vigilante rather than after could definitely work in your favor. He's still probably not quite as useful as Damian, but that's why he costs a little less.

As a note, the synergies section of the OP says that Jason rolls an additional attack die when adjacent to a Vigilante, which is untrue (accidentally copy-pasted from one of the normal Vigilante Sidekick figures, probably).