Xerces has many applications and libraries that give additional functionality to the parser. XQilla, for example, allows for XQuery and XPath implementation.
Xerces-C++ comes packaged with the API documentation for SAX and DOM. These are two of the most highly used programming interfaces with XML.
Implementation of SAX 1.0/2.0 specification.
Implementation of DOM 1.0, 2.0 (Core, Traversal and Range), and 3.0 (Core, Load and Save) specification.
If a website only uses a small number of XML, that can easily be handled manually, Xerces-C++ may be considered overkill. All the validation and compliance may not be necessary.
"... parsing speed is about 50-100x faster than Xerces DOM, 30-60x faster than TinyXml, 3-12x faster than pugxml, and about 5% - 30% faster than pugixml"
Ignoring DOCTYPE declarations causes RapidXML to not be a W3C compliant parser. Though it can still successfully parse and produce complete trees of all valid XML files.