That'd easily result in accidental re-votes if a user watches the same video every couple months (and such re-watchers would likely be ones that.. like the video, thereby skewing data away from dislikes over time).

Especially if the extension just sends the vote status instead of only reacting on a press (which'd allow it to send forward a vote done originally on, say, mobile; don't know if it does this, but it seems like a useful thing to do).