Most of your points regarding RT are perfectly valid, and to be quite honest I don't know what the answer is.
If glider pilots wish to fly x-country regularly, then I would agree that holding an R/T license would be desirable - it is not currently essential or mandated. Whether EASA will change the rules in the future is open to debate, along with the same debate surrounding gliders being fitted with transponders. There is no moral argument against the use of transponders for instance; the practicalities of supplying sufficient power to a transponder on an aircraft that does not generate its own electrical power, is the practicality that must be overcome on this one.
As it currently stands, many glider pilots are happy to fly within the local gliding airfield area, and rarely fly or venture cross country (or until they have progressed to a point where this is something that they wish to do). Consequently, for these pilots there is no real need to know more than the basics to operate from their local field, which for the majority of clubs in the UK, operate on one of the dedicated gliding R/T frequencies. I fly regularly in the US in open FIR, and the same rules apply with respect to a ‘radio operators license’ there.
I for one, got my R/T license before my PPL, and subsequently am more than happy to advise local ATC of my position when flying x-country. Equally, class D zone transition or penetration requires the use of non-gliding assigned R/T frequencies, and therefore holding a valid R/T license is a prerequisite if you wish to fly in those controlled airspaces which gliders are allowed to fly.
If you want to see the current laws and rules pertaining to gliding, with relative extracts from the ANO then go here:
BGA Laws and Rules 15th Edition