This topic has been kicked around all over for at least a decade. It's kind of hard to read a computer programming magazine, third party documentation, technology article site, third party developer mailing list or the like without running into it.
Inevitably someone always mentions how some Operating System has undocumented APIs, and the creators of the OS are using those undocumented APIs, but telling third party developers not to use them. There will always be the rant about unfair competition, hypocrisy, and other evils.
Large targets are Microsoft and Apple, for creating undocumented APIs that are used by Internet Explorer, Microsoft Office, Safari, iPhone development, and other in-house applications. To the detriment of Mozilla, OpenOffice.org, third parties, and hobbyist developer teams.
If you wrote one of those articles/rants/complaints, or agreed with them, I'm asking you to turn in your Geek Membership card now. You're too stupid, too idiotic, too dumb, too nearsighted, and too vilifying (add other derogatory adjectives you can think of) to be part of our club. You have no business writing code or even discussing programming.
Operating Systems and other applications that communicate via APIs provide public interfaces that are supposed to be stable. If you find a function in official public documentation, that generally signifies it is safe to use the function in question, and the company will support it for the foreseeable future versions. If a function is undocumented, it's because the company doesn't think they will be supporting the function long term.
Now new versions which provide even the exact same API inevitably break things. It is so much worse for APIs which you're not supposed to be using in the first place. Yet people who complain about undocumented APIs, also complain when the new version removes a function they were using. They blame the company for purposely removing features they knew competition or hobbyists were using. That's where the real hypocrisy is.
As much as you hate a new version breaking your existing software, so does Microsoft, Apple, and others. The popularity of their products is tied to what third party applications exist. Having popular third party applications break on new versions is a bad thing. That's why they don't document APIs they plan on changing.
Now why do they use them then? Simple, you can write better software when you intimately understand the platform you're developing for, and write the most direct code possible. When issues occur because of their use, it's easy for the in-house teams to test their own software for compatibility with upcoming versions and fix them. If Microsoft sees a new version of Windows break Internet Explorer, they can easily patch the Internet Explorer provided with the upcoming version, or provide a new version altogether. But don't expect them to go patching third party applications, especially ones which they don't have the source to. They may have done so in the past, and introduced compatibility hacks, but this really isn't their problem, and you should be grateful when they go out of their way to do so.
So quit your complaining about undocumented APIs, or please go around introducing yourself as "The Dumb One".