This one makes even less sense than proprietary APIs- there is criticism of KML claiming that it is a proprietary format. I suppose the comparison is to GML, which is supposedly a non-proprietary format. In what sense is KML proprietary?
- It's an open format. Published on the web. Documented.
- There is no use restriction on the format. Other vendors and developers are encouraged to use it.
- It is widely used and implemented by a growing variety of vendors.
- Suggestions and comments on the format are accepted via a public forum.
- Even the compressed binary format (KMZ) is just using the common zipfile compression algorithm, which has implementations on every operating system and modern programming language, rather than something that you need to buy a seperate product to use.
It would appear that the only salient difference in proprietary-ness is that KML was developed by a single company (albeit with input from others), whereas GML was developed by a standards committee. The salient difference in the marketplace is that KML is usable and hand-editable, whereas GML is rather too complex for use without tools. In contrast to what one might expect, the standards committe developed format requires tools to create, whereas the one developed by Keyhole does not. Then again, looking at the history of the OGC, they were primarily pushed forward by the other vendors attempting to cooperate in opposition to dominance in the marketplace by ESRI and ESRI formats such as the shapefile and the arc .e00 formats, which had become the accepted interchange formats for vector data. This infighting has led to OGC standards in most cases being worse than standards which have been defined by individual vendors.
So far they have only been able to come up with "superset" type standards which have been overly philosophical in their approach- not designed with implementation efficiency in mind. I think there is a something to be said for formats that have been proven with a high performing implementation. Think of a reference implementation, such as Apache Tomcat now is for the Java Enterprise Edition for the Servlet specification. It is possible to create a poorly defined or bloated spec such that the implementations are going to be burdened by poor performance.
The proprietary label should be reserved for those formats which are protected by copyright or obfuscation and don't allow for open use- think of the various DRM music formats offered by Apple and Sony- not an XML format that is documented on the web. And oh yeah- ESRI is supporting KML now too- if you need any more proof that it's not proprietary, you must work on a standards committee. I've been there, I'm not going back.