Yeah, I also fear the song will indeed and inevitably become too repetitive if it keeps happening, like it likely will before the show is over. It would be better if she had a few songs, at least. It would be hard to believe this particular song has anything to do with the rain magic; I'd rather think it's merely something that she likes and that gets her into the right mood for the magic to happen. Alas, it's all too common for these kinds of plots to stick to the exact same song time after time. Maybe it's simply a question of the budget, who knows. Maybe only Key with their multi-talented Maeda Jun is free of such inconveniences.