Consider a query similar to the following.
email == 'email@example.com' && (lastlogindate > today - 5 days || newmessagedate > today -1 day).
Which start key do I advance, one, both? As a client I would have to iterate over every field in the expression tree to determine what my start key should be for two index clauses. While this is not impossible, this becomes very complex for large boolean operand trees. As a user, this functionality would provide a clean interface that abstracts the user from the need to perform an analysis of the previous result set and "diff" it with the expression tree provided. I'm not saying it's an absolute must have, but it would certainly provide a lot of appeal to users that are utilizing Cassandra as an eventually consistent storage mechanism for web based applications once union and intersections are implemented in Cassandra.