Last time I talked to Youtubers about this, they acknowledged that there are several important things that youtube-dl can do which the official YT API cannot; the biggest feature is that youtube-dl can archive Creative Commons-licensed footage.

Youtube does occasionally attempt to break CLI tools. get-flash-videos has been beaten down hard by JS-powered authentication, but youtube-dl emulates the JS player. Ads are still broken in youtube-dl, although strangely nobody seems to want to fix them!