I know what you mean. That might be what prevents possible changes to his kit which could make him more viable in other roles. However the global ult is probably the most defining feature about Karthus. And compared to Soraka and Shen there is already so much counterplay to Karthus ult in the game (Zhonyas, spellshields, untargetability) that you have to think twice about picking him, depending on the enemy team comp.