Rustle, my comment about maybe having to stop squawking in some areas was based on what happens now (allegedly) in Holland, as I quoted above:
"The ironical thing is it is MOST STRICTLY FORBIDDEN to squawk below the schiphol tma........? "
WHC, TCAS works with Mode C. I have read elsewhere that the TCAS unit in an airliner (say) scans 30 degrees above and below. If true, it won't "see" you when you are 23500 feet below except when you are too far away to cause an "advisory" (instruction to the airliner to take avoiding action).
We could do with some expert posting up just what Mode S brings extra to the party. My understanding is that it would carry info additional to position (Mode A) and height (Mode C) such as registration, make and operator of airliner, whether they need extra baggage handlers or have had a medical emergency and need a midwife or a doctor, etc. - none relevant to TCAS.
For gliders I suspect it would just transmit that "this is a glider", and possibly the serial number of the transponder and the fact that the glider does not itself have TCAS so won't be telling the glider pilot anything to resolve a potential conflict, leaving it all to the airliner (or whatever) to do that.