Never said leading is only about scoring, but how many team leaders were defensive stalworts that couldnt score in the clutch, led their teams to NBA championships? I don't see Mutumbo, Rodman, or anyone like that being the team leaders on championship teams. They may be extremely key, but none of them have ever been looked at as the team leader. The closest to that was KG in 08 as you mentioned, and that is a ONE TIME case, and still, Pierce is often looked at as the leader of that team, every bit as much as KG was.
There is one thing that every championship team leader has in common... they can all produce points under pressure. That is the ONLY thing they have in common, which obvously speaks measures to the importance of being able to score under pressure. Some are also great passers, some are also great rebounders, and some are also great defenders. Some are even all of that. But ALL of them can find ways to produce points under pressure.
If I am building a team, I will ALWAYS build around a guy who can score and produce points consistently under pressure, before building around a guy who defends, rebounds, but cannot produce points consistently under pressure.