Is this arguing in favour of Spotify?

> buy 13 songs a month, every month, for a year, and see how big your library is.

Sure. And when I had Spotify I added about that to my liked songs every month.

> There is a compounding effect

That doesn't sound compounding, it sounds like a linear increase of 13 per month.

> for convenience of being able to listen to a new song once or twice and never again.

Yes, that is the advantage of Spotify and other steaming services. I can listen to dozens of new songs a day, picking out my favorites while being exposed to new music allowing me to find more that I like. And if I don't like it, meh, didn't cost me anything.

A library of 300 songs is tiny. I guess you don't listen to much music? Back before I lived in the city 300 songs would have lasted a week, I'd statistically hear a repeat on the drive home on the first day.