Purging the old sessions in Cassandra would be nontrivial. Moreover, as Cassandra doesn't give you consistency, it's a very bad session store.
2009/9/15 Matt Kydd <email@example.com>
We need to persist the sessions and associated shopping baskets /
activity summaries somewhere and Cass seems like a good fit, without
the restrictions imposed by SQL there would be less necessity to purge
Also seeing as session data are typically very small (If they're not, you have more problems), the motivation for storing them in Cassandra would be little.
Why not use a conventional database with some redundancy solution - you'll get consistency and for the volumes of data that a web site - even a very busy one - has in its sessions, it won't be a problem.