Winter Wrap Up wouldn't be near as memorable without the song. It would still be a pretty alright episode. Actually, it's a pretty good Twilight focused episode. It is a little lacking in immediately memorable lines, but Rarity's bits are hilarious, as are a few other scenes. It would definitely be mid-tier without the song, I think, but it would still be a good episode. Certainly better than the admittedly bland Owl's Well that Ends Well. For the record however, I don't think that's the worst episode either. Instead, I'd probably go with Call of the Cutie for that.