Everyone seems to be overlooking something that was actually mentioned in the article and I think seems like a pretty obvious reason: extensibility. In a wave,

Everyone seems to be overlooking something that was actually mentioned in the article and I think seems like a pretty obvious reason: extensibility. In a wave, people are constantly updating things all over the place. The ability to show when and where on the wave updates are taking place would, IMO, clearly be a useful feature worthy of implementing a custom scroll bar.