org.jboss.tools.jst.web.kb.kbbuilder is JBoss Knowledgeable builder. This builder gethers information for code assist. Without this builder you will still have code assist working but some info won't be updated if you change resources. It's exactly what you have (see the problem with updated attributes in your XSDs). This builder is added when you enable Seam support or add JSF capability to the project.
So, is there any way I can get the updated xsd file to be applied to my content assist if I can't get this builder working?