The problem is sifting out the content
As a prior poster pointed out, the "useful" videos on YouTube are far outnumbered by the "useless" videos out there. Sure, it might be fun to listen to, or even sing along with, a karaoke video of your favorite song... but that's what you do at home, not when you're sitting in a cubicle surrounded by your co-workers.
Plus, there are plenty of freeware/shareware "YouTube ripper" apps out there that will allow the corporate users to make the YouTube training videos they need available for perusal without opening the corporate firewall to the main site itself.