Wireshark/Ethereal is useful in a load testing context for:
- examining raw http from manual sessions for when the load tool does not easily identify all of the substitution strings
- to troubleshoot load tool generated sessions (comparing to previous)
I get more use out of it in examing performance outside of a load tool context (though there is plenty of overlap):
- troubleshooting network connectivity/authentication
- identifying time proportions among nodes in a distributed application. If user reponse time is 10 seconds, and I can point to 8.5 seconds on say, the data tier, I can use that to direct troubleshooting/optimization.
- Examine whether a network transfer is efficient; are file reads pulling back small chunks of data? Is a database query fetching one row at a time? And so on.
- Detect when bandwidth is sufficient (flow control messages)
Don't know if that helps, but the tool does not generate load, so you may need to be more specific.