My reading of the sessions spec suggests that implicit sessions are subject to pooling, just as explicit sessions are. However our current implementation of with_session ends implicit sessions as soon as the operation using them finishes:

    def with_session(client, options = {})
      session = get_session(client, options)
      yield(session)
    ensure
      session.end_session if (session && session.implicit?)
    end

Other code in the driver maintain implicit sessions for more time, for example cursor iteration, but those are still not pooled.