IMO, having a hobby that's completely unrelated to your profession is critical, especially for people who do any kind of "knowledge work" for a living. Doing something entirely different gives your brain the time and space to process things. It's sort of like how people say they have their best ideas in the shower/when out for a walk/etc. It's not about splitting time between what you enjoy and what you don't - it's more that just doing one thing all the time isn't particularly healthy and can be an active hindrance to doing your best work.