
Anyway, here is my personal belief (take what you want from it).
Watermarking can either have one of two effects:
#1: Be generally annoying and unsightly.
Most watermarks that we come across are hideous. They take away from the overall content and message of the video. Perfect Example:
This is one of my absolute favorite videos. I've watched it more times than I can count. Yet, it hs this hideaous watermark in the bottom right corner that turns it horrible. It's one of the reasons why I haven't got it in my top 100 videos.
HOWEVER
#2: A Watermark is tastefully placed and appropriately used
Another one of my favorite videos. Instead of having an unsightly watermark throughout the entire video, he has:

To me, that is a perfect execution of a watermark (it lasts only a few seconds, and is hard to spot). It's simple, it's subtle, and hardly anybody can locate it. It does its job, appeals the audience, and saves authors from their work being stolen.
Anywhoo, I digress.