Spring provides a higher level abstraction for you by eliminating the scaffolding code you need to write. For e.g. in case of OXM you will be working with Marshaller
and Unmarshaller
abstractions irrespective of the underlying implementations uses (XStream, JAXB, Castor, XmlBeans etc). Moreover it lets you use DI for injecting marshalling/unmarshalling services to your own services. Another advantage is consistent exception hierarchy irrespective of the underlying implementation. All these are well explained on their reference documentation.
If you have very simple needs and doesn't already use Spring then I suggest you stick to JAXB that comes with JDK 6.