I think it should be a case by case basis.
My dad has been a mechanical engineer for almost 40 years and I struggle to think of anyone who enjoys their job as much as he does. Yet I have never once seen him do any personal "side projects" that are in any way related to his day job.
The side project concept is fairly unique to digital workers, especially software engineers simple because we can. Noone asks an accountant or a secretary for side projects.
Not to get into a debate, but there are tons of people who don't like what they are doing. But they are still good at it. Sure they may not be truly happy, but they are good at what they do even if they don't like it.
I can understand a false "expert" delusion, but have you literally heard someone say they'd never work on something unpaid because it's unprofessional?
Occasionally however I run into people who have absolutely no side projects and have been doing the same stuff for 10+ years. I'm a Java expert (or .NET or C etc.), they claim. When I ask them if they've tried anything else besides that one thing they've been doing over and over for a decade, perhaps a side project, a toy product, some contribution to OSS in the new paradigm they apparently really want to work in, I get responses along the lines of "I'd never work on something I'm not being paid for, I'm a serious professional that's irresponsible".
Er.. ok. So I'm supposed to hire you, take a huge risk not knowing if you can pull this off or not, train you for a year to do something you have 0 familiarity with, and then you might peace out right about the time when I'm starting to recoup the onboarding costs? I'll pass. Candidates who already demonstrate a lot of interest, side projects, and desire to learn outside of their 9-5 will win any day of the week with me.