I have been in music for some time, and this is almost exactly what great musicians do. There is definitely a time for activities that are ‘neccessary’ or ‘good practice’ to spend time on, and then there is a large amount of time they almost subconsciously spend in their field: they just enjoy having a beer with friends (at a jam session), having a chat (about music), hanging back (with their instrument), waste time on YouTube (watching poor quality phone recordings of musicians they look up to). Some don't think about anything else. Some have other interests. But I cannot think of one great musician that doesn't touch music when they're ‘off from work’.

I played a wind instrument for 10 years, ending in senior year of high school. I got quite good (if I do say so myself, lol) and had the opportunity to learn from a few excellent musicians in private instructions, chamber music, and the like. I have to say, I agree with the OP. The best musicians I knew critically listened to different pieces of music in their spare time, reflected on their performances, and so on. From what I could tell, music permeated every aspect of their lives, even if they weren't spending their free time practicing technical fundamentals. Their love of the art was such that I don't think they could stop that if they even tried to.

I think having someone who does their coding at work works better for people later in their career, or who have at least done a good solid couple of years of development professionally. For those early into their profession, I usually recommend people dump as much time as they can while they can to get to that point faster, that way when life changes such as they decide to have kids, or focus energy in other passions instead, they are better prepared to make the transition.

In addition, there is a real danger early in a developer's career that their resume becomes a signal of being a weak developer if they don't put in the effort to progress - I have seen this happen to multiple developers who did not take seriously the need to reach a certain level.

Now to tackle the analogy directly, there is a lot of competition for positions in the music world - musicians have to work pretty hard to get stable jobs, or carve out positions of prestige. The most successful musicians I know do it very often, whether it be arranging songs, practicing their musicianship, and immersing themselves in as many musical communities as possible.

They don’t have to actually code outside work. I scratch my coding itch at my job.

But I read all sorts of technology and programming stuff just because I’m interested in it. And that knowledge ends up coming in very useful.

I’d rather have someone like me than someone who does their coding at work but never reads anything that they weren’t assigned to by their job.

To use your analogy, would you want to musician who worked 9 to 5 but never listened to any music outside of their job?