Downsides

• An XML ontology, or a semantic net is just a declarative structure.
• They don't do anything.
• There are more sophisticated knowledge representation schemes, like CyC and rule based systems, that include process.
• So, the system is more flexible, but it may be harder to maintain.
• I think XML and related declarative ontolgies are most widely used.
• However, other types of systems are used, and it's not clear how this field will advance.