My opinion, I don't believe in completely cutting students off from YouTube as they will be missing out on a valuable resource. Having said that, teachers obviously will need to be prudent in vetting content. Having students view videos in Classroom helps to cut out the distractions that come with a typical YouTube page, as in the related videos and advertising. Embedding a video in a Form also serves the same purpose. Of course, students can still click on the "YouTube" button at the bottom of the video and view the video in it's original YouTube page. This should lead to conversations on digital citizenship and appropriate online class behavior. The teacher should also engage in strategic monitoring of the class screens. 

Bottom line, students should have the opportunity to choose to do the right thing, not have that be the only choice.