I think Atlassian would actually benefit from investing the time needed to properly document the backend API for Jira Software. The reason is this:
When no official documentation exists, third-party developers will try to figure out how things work for themselves. Because the Jira backend isn't sandboxed, many (most?) internal Jira implementation details are fully exposed to third-party code. So not only is there a risk that information circulating on the forums is incorrect (or makes use of Jira internals in incorrect ways), there is also a risk that people propose solutions that make things more difficult for Atlassian in the future.
For example, I have seen several recommendations that suggest that we should extract information about sprints from (what I assume) is internal data stored in a custom Jira field. Now that this practice has been established, any change Atlassian makes in that internal data format will immediately break all plugins that rely on it.
I cannot help but think that such a situation would hurt Atlassian too, and not only third-party developers. If, at some point in the future, Atlassian really does need to change that data format and they wanted to avoid the break-all-plugins problem, they would have to implement a workaround that is likely to be more complex and bug-prone than if the internal data format had remained hidden.
One way to look at it is that the lack of official documentation is implicitly causing third-party developers to add to Atlassian's technical debt.
Jira 8 has come and is almost on it's way out and now Jira 9 has been released and still no updated docs?
For shame Atlassian, you were doing so well and things are starting to slide for way too long now.
As someone trying to get into the Jira plugin game for internal Jira needs it's hard to learn anything. I NEED HALP!!!