ACW335
The transponder requirement isn't just for radar; it is also for TAS/TCAS.
Fortunately a transponder good enough to be seen on TCAS requires far less power than the standard sort.
The standard counter argument is that en-route mid-airs in GA are extremely rare, and GA TAS/TCAS is very expensive (a £20k+ option on most new IFR planes) but airliners have it and it will only take an incident involving an airliner and a glider... This summer I have seen gliders which were definitely a few hundred feet in the 2500ft London TMA (around MID).