I mentioned in my last MLPFiM post that I would elaborate a little more on the Winter Wrap Up musical number. What I really meant is that I was thinking more about the fan reaction.
From the very beginning of the animated MLP canon, songs have been a very heavy part. The original specials both had a number of songs, as did the movie, and the animated series had one song per 11 minute segment. My Little Pony Tales had at least one song per episode, and when the G3 cartoons started being released on retail DVDs, they were also mostly musicals. So why should the new series be any different?
Only, the first episode came and went without any songs, barring the opening theme. I don't think anyone really noticed, or thought about it, really. Even I only gave it a passing thought almost two weeks later, shortly before the second episode aired. I guess that most people assumed that since the new series was so different from the other series, both in story and looks, songs were not a guarantee, nor were they really to be expected. So when the second episode aired, and Pinkie Pie sang her "Giggle at the Ghosties" song, everyone was pleasantly surprised (and amused that the song was lampshaded). The next few episodes came and went, and we found that there would be a Pinkie song more often than not, but always diegetic (that is, acknowledged as being sung in-universe).
And then, the Winter Wrap Up song was leaked. And the fans went wild! It seems like everyone who had a chance to see it loved it, and couldn't wait for the episode it was from to come out. There was speculation that it might be an entirely musical episode, and no one saw this as a bad thing. But would there have been this reaction if MLPFiM had been a song-an-episode show from the very start? I don't think so.
It's like the story that Stephen Schwartz told about when he went to see A Chorus Line, and how at the start of the grand finale, he cynically thought to himself, "Oh, of course they're going to do a kick line." But then they didn't. And the song went on, and they still didn't. And then, at the curtain call, they finally lined up and did a kick line, by which time, he was ready to cheer for it. The lesson he took from this was, "If something is obligatory, it's good to make the audience wait for it." And that is exactly what MLPFiM has done.