For use case please see http://groups.google.com/group/mongodb-user/browse_frm/thread/b9de8d8c90125ddd
The current distinct operation return different from cursor type, the list. The returned list can be large, which creates problem at application level. For example, making pagination become cumbersome, since it requires to fetch the list at every page, rather then using cursor/idx/limit/count operations. The situation get worse in concurrent applications.