FOR seasoned bankers and starry-eyed entrepreneurs alike, doing an IPO, or initial public offering, is synonymous with the very idea of taking a firm public. No wonder, then, that the decision by Spotify, a music-streaming service, to opt for an unconventional alternative called a “direct listing” has prompted debate. Instead of paying investment banks hefty fees to arrange an IPO, Spotify plans to have existing shares simply switch one day to being tradable on the New York Stock Exchange (NYSE). IPOs themselves have become rarer, as startups such as Uber and Airbnb have chosen to raise money through private markets instead.