Meh. Many, many applications have at least two database systems: a caching layer, such as memcached; and a persistance layer, such as PostgreSQL. Twitter's basically just using Cassandra in place of memcached. This doesn't strike mas a dangerous or bizarre architecture.
"Twitter's basically just using Cassandra in place of memcached"
Not sure about that. They also said they are using it for analytics. Actually, since Cassandra is persistent to disks, using it as a memcached doesn't sound right.
