So no. He's not useless. He just suffers from the same gear issues as all the other companion tanks, with the added problem of aesthetics dictating that it would be beyond ridiculous to stuff him into Jedi robes (or Trooper armor).
As a dedicated tank myself, I don't particularly need Scourge to take any hits, ever - he's mostly there to add some token DPS when I've got Kira off crit-crafting orange robes (he despises me so he's useless for crew skills) - so I stuck him in light armor. None of the consular robes are perfect for him, but some don't actually look half bad. He also looks good in the Red Reaper chest, if the hood doesn't annoy you too much (sadly it does me).
But yeah Rusk is pretty bafflingly useless. Hard to gear, since you can't craft for him or give him hand-me-downs. Inefficient in combat since unlike Kira, none of his abilities synergize with yours. Even his story is ill-fitting, it feels like he took a wrong turn on his way to the Trooper's Chapter I.