Yes, it only damages people, but it is a targeted global ultimate as well, it can kill people that just barely got away that no other abilities on the team can reach, help you snowball in lane by getting kills and assists from side lanes without having to give up farm from roaming, soften up an entire enemy team to give your team a huge advantage in health before a team-fight. It is definately not his strongest ability, but it is nowhere near the weakest. And if you honestly believe that his passive is only good for a recast of his w (which he could and should have already done when he was alive since its an instant cast), then you REALLY don't know that much about Karth. His passive is what allows him to build full damage and still dive into team-fights head first to maximize his damage potential. Even if he gets blown up, instantly because he had no resistances, he still gets seven seconds to do whatever the hell he wants to. For seven seconds, all of the abilities, mana, cooldowns, and time spent killing Karthus were just wasted, and they now he has an uninteruptable ult as well. If I had to rank his abilites from strongest to weakest I'd say Passive, q, r, w, e. His w is still a good anti engage or disengage like you said, but that isn't going to win him any fights. It is very good for preventing them or forcing them, but it simply doesn't offer enough to rank any higher for me personally.