Oddly enough, a majority of gliders (and helicopters) in Europe, and a rapidly increasing number in UK are equipping with an effective anti collision system called FLARM.
www.flarm.com It also gives obstacle warning.
TCAS doesn't hack it for the mode of flight (in the Alps it would be continuously active and hence ineffective, or for the panel space/power considerations.
Of course, this is not compatible with the CAA's wish to equip all gliders with Mode S which will not give them collsion warning against the majority of their own risk, and is several times more expensive.
And when the USA shortly goes ADS-B, compatibility issues become even more questionable:
For general aviation aircraft the FAA has chosen a Universal Access Transceiver [UAT] surveillance link using 978 MHz in the DME band. UAT was developed in the USA specifically for ADS-B operation. (The Europeans seem to be favouring a third technology – there is no interoperability between the three technologies.)
In the light of all that, please convince me I should shell out for a Mode S transponder in my sailplane (where I don't have the panel space or spare battery capacity anyway).