[reading-at-work]

I think the point of the person you're replying to is that there are other methods of assessment which would render plagiarism ineffective - such as in-person presentations and conversations. But, teachers don't have the time and resources to do that with all of their students.

Effective writing is an important skill, so I don't think in-person conversations would replace papers, but from a conversation with a student who plagiarized their paper you could probably tell that they didn't really understand what they "wrote."