Do you know of an instance where a 15 hour CFIG went straight to frequent teaching? If so isn't that more a fault of the club or commercial operation? We get it it would be unsound if 300 hour airplane CFIs were doing 15 hours in gliders then going off teaching, but I don't see it happening. The only place it probably has happened is in the aviation flight academies that operated motor gliders. They must have had enough rules or gray hair supervision to make it work.
Yes I get the indignation cause it could happen and those 15 hour CFIG candidates didn't suffer enough. Personally I'd welcome airplane CFI add on to toss the CFIG in the pile early on, then take the process of using it really slow. Toss em easy stuff occasionally while they are recreational soaring and building experience. I encourage private pilot airplane transitions to jump straight to Commercial, not to immediately go out and hustle rides but so years down the road they are one less hurdle from becoming CFIs. As to the horror of creating low experienced commercial glider pilots most decline and do the private add on.