I'd argue it is a bit more common than you think, but probably for different reasons than you would assume. It isn't so much the impact of the side project that gets you the job, but more about networking. If you are involved in an am-pro manner in a given FOSS community by actively participating (code, docs, etc) and speaking (blogs, conferences, workshops, etc), your networking opportunities explode. Anecdotally, this is how I landed some really nice gigs, even if that community (and tech) has all but evaporated now. But if you're toiling away on some project and not actively engaging with anyone that would remotely care, your potential employers very likely won't care either unless they are specifically looking for it.