I think it’s a good letter, especially considering what the target audience is for it: glider pilots.
There is no doubt that using the radio for ATC comms while trying to soar cross-country adds significant workload to what might already be a challenging activity. Practice helps, as does a transponder but most glider pilots are reluctant to engage with ATC unless they really have to, for the above mentioned reason.
Yes, it is good airmanship to avoid possible areas of conflict and to increase general situational awareness by conversing with ATC but we are talking about uncontrolled airspace here, Class G. See-and-be-seen, random separation, FLARM, TCAS, ADS-B, etc. Any sort of service you get OCAS will be by definition incomplete as who knows what’s out there?
Flying a powered aircraft from A to B (maybe even via C!) along a straight line at a consistent altitude and speed is very different from the average glider which operates through quite a large height band, changing speed and track and altitude almost continually, occasionally stopping to circle. Any information about a glider rapidly becomes stale and it is difficult for ATC to help much if they don’t have a secondary contact.
It is very much a two-way process of understanding: glider pilots are getting better at contacting airfields to give and receive traffic information and ATC are improving their understanding of the typical flightpath and workload that glider pilots face, especially when getting closer to the ground. This is good but it still has some way to go, hence the open letter to encourage this sort of activity.