Sure, but how many people involved in a day-to-day job of (let's say) supporting a Rails application in their spare time read up on (let's say) data structures, algorithm design, operating systems, game design, database management systems or functional programming?

In an ideal world everyone does this, in reality I found that professional experience is more conducive to growing as a specialist vs growing as a generalist - conversations with coworkers, dealing with bug reports, browsing StackOverflow and attending conferences are all centered around getting deeper, not broader.