- Timeless. If you have an event and you set the date inside the URL, then after this date has passed, this URL gets less valuable. Either 301 these aged URL's to the current event URL, or make it so your URL's can be "re-used" for future events. Cool URLs don't change.
Where does the user go to find the 2008 event, if his post from 2008 has a URL that now points to the 2013 event?
A link to https://us.pycon.org from a post dating back to 2008 now redirects to https://us.pycon.org/2013. But that is on a domain basis. Hypothetical: python.org/pycon could always show the most current event, with a link to an archive, if you'd want to read the announcement page of the 2008 event, for whatever reason.
If you use python.org/pycon-2012/ and it gets 1000s of links, then you lose all that if you create a new URL python.org/pycon-2013/.
This does depend on the event of course and how bad it is reduced in relevance after its date has passed.
Where does the user go to find the 2008 event, if his post from 2008 has a URL that now points to the 2013 event?