If you structure it well, you can have your cake and eat it. I enjoy many frivolous "side projects" that are merely for research or to learn a technology, but where possible I try and plan for that "play" to then serve as the basis or inspiration for a product or service I can offer as well (at the worst, a fun side project could turn into an interesting piece of content marketing, a prime piece of your résumé, or even something like a class or an e-book).