The movie number is an integer number field so we can't store a youtube link (contains letters), but I could add a separate youtube field. Though that in such case we can also just post the link in the comments.
The reason of the IMDB number/link is that we need to have a number for each movie, so instead of taking "random" numbers (like what is done for movies/clips which do not have IMDB number) I thought it was good to use the same numbering as IMDB to be sure to not list twice the same movie (this already happened with movie clips) and to easily build the IMCDB link from the IMDB link.