I agree for plain documents, but tons of online articles list comments on the same page, so the scroll bar is almost a negative incentive to keep reading.
"I've read this much of the article and I'm only 1/20th of the way down?" [user stops reading, unaware that there's 450 comments and the article is actually pretty short]
I think comments themselves are the “negative incentive”, not the scroll bar. If comments were as informative and well-written as the article, then including them in the total article length is what you want because you would also want to read them.
But, as we all know, comments are usually total garbage. The problem is not that they get included in the scroll bar, but that they get included with the article. We need to fix, or remove comments, not scroll bars.
"I've read this much of the article and I'm only 1/20th of the way down?" [user stops reading, unaware that there's 450 comments and the article is actually pretty short]