Aaron Swartz waxes philosophical on the Jon Postel phrase Be liberal in what you accept, and conservative in what you send.
It seems to me that a well-designed protocol would have only one saying things and require that you use it. After all, there are two types of interfaces: good interfaces and user interfaces. (There is actually a third type, as demonstrated by XML: bad interfaces, which are neither good for computers nor users. It never ceases to amaze me how XML is bad at everything.)
I can’t say that I agree, but it is definately a point of view. I think that there’s lots of hype around XML, but it does serve its purpose. It may not be easily human readable, not easily computer readable (or without headaches), but it might be the lowest common denominator.