Google will be releasing an open-source KML library in C++ that implements and tracks the standard as it progresses.
I can see two intended audiences for this library; kml content creators and content consumers. I just don't think it makes sense for either of them. For kml creators, why would you need to interface with a C++ library in order to create kml files. The answer is, you shouldn't need to. It's kind of like my post on Dreamweaver, if you know what you are doing it just gets in the way and if you don't know what you are doing it is way too complex. A C++ library seems like overkill to write out some xml text. I guess it could keep track of external and document-wide styles? Big deal.
If you are a kml consumer then it makes a little more sense to use a library in C++, but not much more. Using external libraries requires you to build a bridge between your code and the library concepts. So the libkml will be dictating what types of entities you support and how they are interrelated within your code. This is restrictive on how you would develop your internal classes by forcing you to make a class structure identical to the libkml structure or you could try to build a conceptual bridge between your internal structure and the libkml structure in order to be compatible. Once you have either one, why would you need an external library just to parse the XML entities.
By providing a reference library it allows developers to more easily keep up to date with KML without having to maintain their own library and track standards changes.
So developers won't have to support any changes in the standard if they are using libkml? I guess it sounds more like it's for the kml content producers.
I guess an alternate explanation is that they are trying hard to make it seem like the standard will be truly open. Of course I'll take a look at it when it comes out to see what it's all about, but the whole concept seems like an exercise in futility.