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.
Compare: /search-engine-ranking-factors-2010 /search-engine-ranking-factors-2011 /search-engine-ranking-factors-2012
or a catchall:
/search-engine-ranking-factors
Showing the most current one. An URL like above will collect links from all the years, instead of spreading it out over multiple years.
P.S.: If you register pycon2013.org then domain squatters might register pycon2014.org pycon2015.org etc.