HTML and the concept of a URL are what made the web possible.
Efficiency comes from constraint.
That is, it is considerably less constrained and, as a result, considerably harder to make work in the real world. This is not necessarily a bad tradeoff, but it is critical to understand what XML brings to a given application to decide if the extra problems and complexity are worthwhile. Your data is described using tags that describe what each piece of data is. XML is the Web's language for data interchange and HTML is the Web's language for rendering. Applications that attempt to distribute a significant proportion of the processing load from the Web server to the Web client.