Transmit-only mechanisms are definitely the best way to go to reach the extreme minimum in low power levels. The problem with transmit-only systems is that they are difficult to use in most real-world situations. Once you have deployed a transmit-only sensor, you are stuck with the configuration that was hard-coded into it. There is no way to change how often you want the sensor to report, how and when it should trigger, and any other parameters and requirements that may change as the project develops - encryption keys being one example.