Yes, why new songs on a special holiday? Wouldn't you want some traditional songs that everyone knows how to sing already? To me this is way to make families sing these songs at home. Practicing the song is further brainwashing for parents and children. Many will do it because they won't want the congregation to sound terrible singing a song they have never even heard.
I think you're on to something. They got rid of the bookstudy (for whatever reason, I know there's a few theories on why, none of which actually involve gas prices) and now I think they're struggling to replace the extra indoctrination with something. Sure they've given you the family worship night as a requirement to fill the void, but without the pre-planned content that JWs are so used many don't keep at it. If they give just a little content and "encourage" them to do it during the family worship night (that you're definitely doing regularly, right?) then it gives them more of a push (and a guilt trip) to actually self-indoctrinate more regularly.