Even if that in-field experience is open source.

Well, IT is much more than writing code. Open source code can act as a portfolio, and can be useful for an interview (although it can't answer many of the questions an interviewer will find important). But code can't show your experience/knowledge of a certain OS, or hardware. I do have code I can show, but I also have solid knowledge and experience of several major Unix platforms. Which, for many jobs, is far more important than any code I have written.

I want to know what you have done, and how that makes you a better candidate for the position I have to fill. Certification rarely answers that question.

Yet it's often the only fysical evidence a candidate can present. I would never hire someone purely based on the existence of certification. But if I have to pick between two candidates, all other things being equal, the fact that one has the certification, and the other doesn't, might drop the scales in favour of the candidate with certification.

My advice is, if you can get certification without paying an arm and a leg for it, go for it. Noone will not hire you because of having a certification, and it may help you to secure a job. If only for getting through the HR filter. Or by being noticed by a recruting agency.