And the one you didn't list....it's a way for skaters/coaches/choreographers to get around copyright...if they use the original then they run the risk.
The copyright issue was alluded to in #1 and #2:
1. More affordable: All recorded songs are covered by copyright, but it often is less expensive to use a cover by an unknown artist than the high-profile original.
2. Watchable on Youtube: Depending on the terms of the copyright, a skater might be allowed to use a version of a song in live performance but not be able to post it online for wide access. A cover version is usually less restrictive and/or less costly for online sharing.