It can be useful to know the set of in-use partition keys (storage engine row keys). One example given to me was where application data was modeled as a few 10s of 1000s of wide rows, where the app required presenting these rows to the user sorted based on information in the partition key. The partition count is small enough to do the sort client-side in memory, which is what the app did with the Thrift API--a range slice with an empty columns list.
This was a problem when migrating to CQL3. SELECT mykey FROM mytable includes all the logical rows, which makes the resultset too large to make this a reasonable approach, even with paging.
One way to add support would be to allow DISTINCT in the special case of SELECT DISTINCT mykey FROM mytable.