Wouldn't have to be open source work, just work you could show.
It's tough to hear, but the goal of a new employer is not to give us what we deserve for working hard and being skilled.
The employer's goal is to maximize their chances of hiring someone good.
If a hiring manager has two people that seem about the same but they can see the code one of those people wrote, it's a no-brainer to go with the one who has a portfolio.
Agree with you that some lame open source patch doesn't matter. But if someone's done significant work in public, or even has non-open-source code they're able to share, ignoring that would be an insane choice for the hiring manager to make.