TCAS has value for everyone flying at low level.
My experience was flying a legal and notified low level flight in my civil aircraft, TCAS alerted the presence of two aircraft which were not immediately identified due to the visibility and their camouflage (2 x Tornado). TCAS enabled awareness, a direction in which to focus attention, and after contact confirmation that safe separation was being maintained.
I doubt that they saw us, more usually the presence of a 4 engine airliner at 250 ft gets a wing wave or a quick formation / fly-by with a ‘what are you doing down here? look’.
I would have thought that the military would embrace benefits of TCAS to alert the presence of traffic. In peacetime, another aircraft is just as much a threat as a missile / gun carrying fighter in wartime, either one hurts if you are hit by them.
Similarly, a wider spectrum of civil aircraft should consider the cost/benefit of TCAS; flying is expensive, but so is dieing.