WS-Eventing Section 4.1 states that:
Clients that wish to isolate expressions from the effects of any changes to the namespace prefixes in the containing SOAP message are advised to construct expressions in a manner that avoids the use of namespace prefixes. For example, use an expression such as "/a[namespace-uri()='http://www.example.com']" not "/ns1:a". 

This paragraph implies that xpath expression:
/a[namespace-uri()='http://www.example.com']   

is equivalent to:
/ns1:a

However, it is not. The first expression will evaluate false on XML:
<ns1:a xmlns:ns1="http://www.example.com" />

whereas the second one is true (given the proper namespace context).

The XPath expression equivalent to the second one is:
/*[local-name()='a' and namespace-uri()='http://www.example.com']

Proposal:
Use the equivalent XPath expression instead.