You're comparing extremes. The well-rounded person without side-project might just be as capable and experienced as the candidates he's competing with. If that's the case I'd always rate well-roundedness and interests outside of the immediate scope of the job more valuable than a long list of side projects.
That's great, someone is a better rounded individual. That's nice.
But if that well-rounded individual applies for a React position but has zero React experience, let alone any React project in their portfolio or CV, then that well rounded individual will be quickly brushed aside on the first round of the application process.
But at least he's well-rounded.