The Nepomuk Service should also have a Telepathy Observer as part of it, in order to store details of people we interact with through Telepathy who are not in the Contact list in Nepomuk.
As I recall the rationale is to keep track of contacts on IRC or strangers in a jabber conference. I see no reason to do this. If they're not in your roster, why would you want to keep track of them. We'll just flood nepomuk with garbage. Martin?
Technically it fits the idea of semantic computer usage, practically I don't think we need this right now. We've disabled IRC, we don't store logs in Nepomuk (yet) and so there's no real usage of those contacts, they would be just sitting in your database. Might make more sense when we do more semantic stuff. For the record, nepomuk is already filled with /lots/ of...well..."garbage" data - we feed mp3 artists as contacts, we feed all email addresses from all people that ever sent you email etc. But this is different from the email case, because you can send email to anyone (so email addresses are useful), but chat only with people in your roster through Telepathy.