It really isn't. I can tell you that personally I've already been contacted and hired a couple of times exclusively due to a side project I have.
> But lets just say that I would have to work full-time on a side project
You don't. You only need to have something to show for. You are the only one whose input matters and you are free to choose whatever you feel is best for you in terms of learning experience and portfolio. No one cares if you work on it for 14 hours a day or 15 minutes each month. What recruiters do care about is that you have something substantial to show for when they ask you if you have any experience in X.
> (..) but according to the grandparent comment, their skills are suspect because they haven't spent their afternoons writing dead end projects for fun.
Those skills are obviously suspect if they claim prowesses that they have no way to corroborate.
What? Do you believe people don't lie in job applications? Do you believe no one ever said in job interviews that they were very skilled at X although they never in their life had any contact with it?
Why would someone have no way to corroborate their industry experience? I can show you thousands of lines of code I wrote on the job. Even with an NDA there are ways to show your achievements.
Anyway, I am not saying side projects don't get peoppe jobs, that was never my argument. My argument is that talented developers don't always have side projects all the time. Some of them are already highly skilled and work in challenging enough environments that they continue to learn anyway. The grandparents argument was that anyone without side projects is suspect, that is demonstrably false.