The Challenge of Open Systems

Expecting the unexpected While XML is useful in closed-system environments, where all possibilities are accounted for in advance, it was explicitly designed for the Web, where the only known probability is rapid and sometimes random change.
The recipient reigns supreme In any messaging situation, the recipient will see the message through its processing. While senders and recipients may have relationships outside of the message which govern how that processing takes place, the message will be read, interpreted, and possibly rejected by the recipient on terms which it sets.
Sometimes avoidable We can have committee meetings and published protocols for some kinds of information handling. This isn't an unusual process for large-scale information interchange. Developers who want to make their systems scalable (read centralized) typically insist on such agreement.
Sometimes unavoidable In smaller-scale cases, there may be little incentive to standardize formats. Document exchange in particular is pretty simply handled without pre-agreement, but there are a number of generic messaging approaches where establishing agreement on all aspects of a message is seen as a higher-cost solution than managing anarchy.