One of the university degrees I did (effectively an arts degree) involved doing a lot of essay writing as part of the major. While I don7t really use the field knowledge I studied in the degree I credit my reasonably ok writing ability to all essays I wrote - which I have put to good use in writing technical reports and user documentation that no one else wanted to write because it was seen as beneath them.
Even with the ability to catch copy-pasters with tools such as turn it in, there is always private essay/thesis writers you can hire (there was an article from one of these writers sometime last year).
I just have to trust that despite a fancy degree, the cheaters get caught out in the place where it really matters - the workplace.
Not that I think "standardized" tests are any better. Just that increased awareness of this kind of effect on perception might decrease its impact.
If I write extremely poorly, the pain of reading through it taints what data exists. The irritation lowers the perception, which lowers the grade. It's a pretty well known phenomenon, lots of studies supporting it, probably most classically known as: sitting in a hard chair makes you more "hard" in your dealings with people.
Note that your example with the presentations only supports the belief that the delivery of the proof-of-learning matters. Not that there's a way to fix it, of course - awesome results are awesome, and should be encouraged. It's simply that I wish more professors were (more acutely?) aware of it, so it would (hopefully) have less impact. I would think that there would be a bit less cheating if delivery didn't matter.