It's also vague to the point of meaninglessness considering all the various kinds of technologies and depths within those technologies one might be toying with in their spare time.

I would argue that the general intuition that many programmers have about people who code in their spare time is simply a too-narrow view of the picture. It's more about people who are really good at LEARNING, and people who code in their spare time tend to become very good at self-teaching new things and figuring out how different systems work and interact.

A developer can be good at learning as long as he or she is practicing learning something in their spare time, but I don't think it has to be a side project (or even technology-related at all) to see a benefit in professional productivity.