1) They are a group entirely funded by academic grants, they do not rely on or collect revenue from Youtube's adstream. So a full reupload should not harm them.

So what? You are still taking away the credit.

2) Links to Youtube videos simply do not work well on Reddit. The lack of attention gathered by the original provides a good illustration of this.

Wut? How does stealing the video possibly get them more attention than not stealing the video?

3) If the video on Youtube is taken down (as often happens through spurious automated copyright claims), this provides some sort of backup, so that it may still be found on the Internet.

Mirror the video, fine. But link to the original.