A lot of my issues about the weird side project fetish come down to the fact that this is an award for the misaligned. I spent pretty much the first decade of my 'proper' career working on ideas that went into our regex matcher (now at github.com/intel/hyperscan fwiw). During some of the more intense periods I could barely take a shower or daydream without having to grab a notebook and start writing ideas down, many of which went into the product. When I wasn't doing that stuff, I really didn't have any energy left for building anything else. Reading these discussions, I often wondered what would happen if the associated startup (Sensory Networks) had tanked.
This isn't hating on people who do side projects; it's just to say that having the time/energy to do side projects often feels like it results from misalignment with your day job. I was happier than a pig in shit writing regex and literal matcher code (or designing algorithms) at Sensory Networks and I'm still that happy doing new software projects at Intel.