Right now, there is a way to delete column with the CassandraStorage loadstorefunc. In practice it is a bad idea to have that enabled by default. A scenario: do an outer join and you don't have a value for something and then you write out to cassandra all of the attributes of that relation. You've just inadvertently deleted a column for all the rows that didn't have that value as a result of the outer join. It can be argued that you want to be careful with how you project after the join. However, I would think disabling by default and having a configurable property to enable it for the instances when you explicitly want to use it is the right plan.
Fwiw, we had a bug in one of our scripts that did exactly as described above. It's good to fix the bug. It's bad to implicitly delete data.