Is having a chapter in an open-access book valuable as a publication?
For the kind of book you seem to describe, I think citations would be to chapters of the book, hence to the authors. 

But pretty much any reviewed and edited publication you have helps you, though some more than others. Since you have done the work, consider the relative merits of having this vs not having it, or someone else having a chapter in your place. 

If you still had the work to do, you might consider where your time and efforts would be better spent, but with the work behind you, the writing effort should pay off over time. If no one else cites it, you certainly can in work that builds on what you did. It gives you visibility with the other authors as well, and can lead to future opportunities. 

I don't see any negatives other than the time spent in writing. And, saying no to the editors might not be the wisest move.