If a library works like it should, then you may never have to look at it - that's great. But if you're working on a team, you'll have to read and understand other people's code all the time, and even if you're not, someone, somewhere, is going to eventually want to make that library do something different.
I agree that, of the two, good test coverage is more important than pretty formatting, but a good code style is going to make the code easier to debug when those tests fail and easier to extend when the library needs to do more. It's a simple matter adopt a coding style that works and stick with it, so there's really no good reason not to do it.
By comparison, it does seem like a trivial thing to obsess over. There are a number of coding styles which are all fine in their own right. I think people should worry less about the specifics and more about the overall intent-to write consistent, clean code. How many blank lines you have after block of code or where you put the opening bracket doesn't really mater in the end, as long as you follow the same structure across the whole code base.
One of the arguments/justifications for the PSR standards is to encourage interop of libraries (to be able to mix/match from various frameworks and projects).
Standardizing formatting for a team/project is not a bad thing, certainly, but if there's competing styles on a team, but they're making good progress - hitting deadlines, high test coverage, good/deep engagement with stakeholders, etc., code formatting is just not something I'd bother enforcing. It may be a later step to go back during a project post-mortem and get the code ready for 'deep freeze', assuming the project is 'done', but how often does that happen?
My perspective is probably a bit different than some here - I freelance, and work in multiple languages for different clients, often concurrently. In a 6 month period I had 2 PHP projects, 2 Grails projects and a Rails project, each running for several months and overlapping. I worked with a different group of people on each project, each with varying skill levels and backgrounds. I adopted my style/technique as best I could for each project, and didn't harp on formatting with anyone, because... it just really doesn't matter. What does bug the heck out of me is someone rechecking out my code, spending time reformatting it, then checking it back in, and counting that as 'work', when there's many many many other issues needing to be worked on.
I agree that, of the two, good test coverage is more important than pretty formatting, but a good code style is going to make the code easier to debug when those tests fail and easier to extend when the library needs to do more. It's a simple matter adopt a coding style that works and stick with it, so there's really no good reason not to do it.
By comparison, it does seem like a trivial thing to obsess over. There are a number of coding styles which are all fine in their own right. I think people should worry less about the specifics and more about the overall intent-to write consistent, clean code. How many blank lines you have after block of code or where you put the opening bracket doesn't really mater in the end, as long as you follow the same structure across the whole code base.