It seems like your point is that if you have side projects, your job is probably not hugely fulfilling. I can sort of go along with that, but that's going to be true for the overwhelming majority of jobs. For every Carmack or Linus, there's hundreds of developers doing necessary but unglamorous and non-cutting-edge work like B2B SOAP services or SAP integrations or yet another J2EE storefront. It's not reasonable to expect that most intellectually curious developers will be satisfied with their day jobs.
Side projects are not a matter of "having an unfulfilling day job". A side project is simply scratching an itch.
That itch could be something that bothers you, or it could be simple curiosity.
I have a very fulfilling day job (I'm a co-founder of an awesome startup, which makes it more of a fulfilling day&night job), but that doesn't mean I won't explore other things, try out new technologies, find something that sucks and make it better, latch onto an idea and build it while thinking "how hard can it be?" (Answer: it can be VERY hard)
Side projects are an outward expression of people who are hopelessly creative and just can't help themselves. The CONTENTS of those side projects demonstrate how organized, focused, and disciplined they are.
