A SOAP message with a timestamp that does not expire is vulnerable to replay attacks.
A nonce is a cryptographic random value that is sent with a message to prevent replay attacks. When a nonce is combined with timestamp (or expiration), each message is unique and is only valid for a certain amount of time, thus thwarting any attempt to retransmit an old message.
[1] Standards Mapping - OWASP Top 10 2004 - (OWASP 2004) A10 Insecure Configuration Management
[2] Standards Mapping - OWASP Top 10 2010 - (OWASP 2010) A6 Security Misconfiguration
[3] Standards Mapping - FIPS200 - (FISMA) CM
[4] Standards Mapping - Common Weakness Enumeration - (CWE) CWE ID 254
[5] Standards Mapping - Web Application Security Consortium 24 + 2 - (WASC 24 + 2) Insufficient Authentication
[6] Standards Mapping - Payment Card Industry Data Security Standard Version 1.1 - (PCI 1.1) Requirement 6.5.10
[7] Web Services Handbook for WebSphere Application Server 6.1 IBM Redbooks
[8] Web Services Security Username Token Profile 1.0 OASIS