In the slides Jesus linked, David Spivak states,

>It offers the opportunity to deeply integrate programming and data.

If the database is written in XML, and the programming language you're using uses s-expressions (such as LISP or Scheme), XML statements can be easily translated into a subset of s-expressions called x-expressions, the above statement of integrating programming and data becomes quite simple.

In fact, the world wide web using Javascript as the scripting language of HTML/XHTML web pages instead of LISP or Scheme, is, in my opinion, a hugely missed opportunity.