It appears that both companies are doing their best to protect the individual users privacy. This isn't about going after individual Youtube viewers for damages - it is about Viacom getting every possible cent out of Google in copyright infringement damages.
From NY Times:
http://www.nytimes.com/2008/07/04/technology/04youtube.html?ref=technology (free membership required)
""In a letter sent Thursday, Google’s lawyers pressed their counterparts at Viacom to accept a more limited set of data. “We request that plaintiffs agree that YouTube may redact user names and I.P. addresses from the viewing data in the interests of protecting user privacy,” wrote David H. Kramer, a partner at Wilson Sonsini Goodrich & Rosati.
In a response, a Viacom lawyer wrote that Viacom was “committed to working with Google” on the privacy issue.""