I'm not saying don't work on your own stuff, but if you're going to invest a significant amount of your life into something, ideally it should also have the potential to earn you some money. Unless you're already rich and your problem is boredom instead of money like for the 99.9% of us.
I'm doing just that. I have a long running project that I built for myself, which was tangentially useful in experimenting various software architecture designs but the key part in this project is that it's "business driven". So if I understood something it's the employer's focus on the business rather than the tendency of developers to get lost in the useless beauty of technicalities.
I seldom build a feature for the fun of it, mostly I just need the damn thing to work. And when I do build the feature, it's almost never the most bleeding-edge aggressive optimized version, it takes me weeks and months to think about an use-case so running getting results in 10 seconds versus 100 seconds is absolutely irrelevant, actually relevant if getting 10 seconds instead of 100 takes me another couple hours of thinking.
Recently I am dabbling with the idea of starting another personal side project and again, it's gonna be something of potential commercial value. No harm combining the pleasant with the useful, in fact being useful rather than useless is what keeps me going.