XML aims to be both human-readable and machine-readable, but manages neither. It's only really worth it if you actually need the complexity or extensibility, otherwise it's just a major pain to map XML structures to any sensible type representation. I've been forced to work with some of the protocols that people like to present as examples of good XML usage and I hate every single one of them.

Fuck YAML though. That spec is longer and more complex than any other markup language I know of and it doesn't have a single fully compliant implementation.