I have to admit that YouTube is not my favorite. In fact, my kids are not allowed to peruse the site unaccompanied by a parent. The possibility of inappropriate content popping up during an innocent search is too great. I would define YouTube as another stop on the Information Overload train that comes rumbling through my inbox more often than welcome. I guess if I try hard I can imagine a scenario where a library could upload a video of an instructional tutorial on how to use the OPAC or something similar and then place that information somewhere near the OPAC or on it's website or Facebook page. However, that might subtract a potential face-to-face encounter with a live librarian from an equation that might ultimately encourage a user to use such services.
At any rate, I found a wealth of information on YouTube where Vimeo produced nothing of what I was looking for. Enjoy the flash mob in Rem Koolhaas' incredible Seattle Public library.