Because if you are trouble shooting a problem, you'd have to go fly with a bunch of other gliders and then come back and see if anything changed.

What's needed is a box that could tell you on he ground that "yes, your flarm is transmitting at an acceptable power level, or no, you have a problem." Similarly, it would be just as important to determine that your Flarm is receiving a signal that duplicates a glider at say a 3km range"