Why this information? Funny you should ask... (Score 1)

The first step is to link the email to a unique person. Once done, any email can be cross-referenced with your telephone records (NSA wiretaps). This information can be used to create a web of interactions. An example where this would be applicable might be (my apologies to all Robert's): Bob got an email from an IP address in Pakistan. Use Bob's profile to create a list of the people that Bob has had traceable interactions with (Bob's web). Now we can overlay Bob's web with other suspects' webs and look for hits.

The time information could be used to correlate interactions with past attacks (9/11, Millenium, etc). For instance, show me Bob's web +/- 1 week of 9/11.

If the information is being used this way, then I can see it as being potentially useful to law enforcement in trying to reduce massive amounts of data into something potentially actionable such as initiating additional surveillance on Bob.

The flip side of this is some spammer sends out email from a server in Pakistan. You happen to get this email. You live near a suspected terrorist. You happen to bank at the same place and order pizza from the same place as the suspected terrorist. You happen to have ordered a pizza on 9/10. You get a one-way ticket to Cuba.