Side projects are mostly important for new grads or students looking for internships, just to show - "look- I can do code that's not just my data structures homework assignment."
The biggest mishires I've experienced were generally students with a whole lot of history in their GitHub accounts. I haven't done a thorough post-mortem on what was going on there, but I wouldn't rule out plagiarism or an ability to make small tweaks but little sense for dealing with a larger, more complex project.
What we found was the way you figure out whether or not someone with a strong GitHub history is going to be a good hire looks more-or-less exactly like the way you figure out whether or not someone with zero GitHub history is going to be a good hire. All focusing on GitHub really seems to get you is the ability to cast an artificially narrow net.
You would think, I've been in the industry 25 years and my side projects are still my biggest sales tools when it comes to changing jobs or joining projects. Makes a huge difference to be able to show that I stay constantly on top of the latest changes and best practices and apis even when my current day job may be using an outdated stack.
