People using an alternative to YouTube are already a very specific minority, and the percentage of those using a plugin on a specific client is even rarer. I wouldn't go so far to call it a ballpark. Or even a rational source.

I would agree with you that the data would be highly suspect. It wouldn't reflect reality at all and should really not be considered.

On the other hand, YouTube likes and dislikes have been a tool for ages, and they were manipulated. It was never more than a silly toy. It was never accurate data to begin with.