As gliders can't always be fitted with TCAS/Transponder/battery consuming devices then we remain reliant on a good lookout/'big sky' theory. The problem with lookout though is that quite often closing speeds are very high - so why don't we simply mandate a lower max cruising speed in Class G? I'd like to propose 60kts as that's the speed that my glider flies most efficiently at. That'll buy us all far more time to see and avoid each other. Also makes a collision in cloud much more survivable, particularly if you're wearing a parachute (which I presume anybody flying in cloud OCAS does as a matter of course).
On a more serious note, I think ChrisN illustrates the wider issue very well - yes there is a small risk of collision but it's much much smaller than other risks.