I've just never been able to understand why the gliding fraternity usually requires an instructor rating to fly passengers
This seems dismal. Is soaring really that hard, that only an instructor can give rides?
Here in the US, many of us earn the private rating in a Schweizer 2-33. Easy to fly glider, very strong, very forgiving, very safe. Both clubs I've been in have higher performance 2-place gliders, like a Grob 103, ASK21, even a duo discus. To fly the higher performance gliders, our club rules require a certain minimum level of experience, and a checkout from an instructor. The instructor does the duty of screening out the people not competent to fly the machines. Indeed, this job starts before the high performance checkout. After all, the instructors solo the students, recommend them for checkrides.
At my club we have a pool of commercial glider pilots for taking paid glider rides. Usually we don't waste instructors on this task, as instructors are harder to come by, and should be instructing. Some of our commercial pilots are teenagers. Darn good pilots too. Some of them are old guys with 1000's of hours, but no instructor rating. But boy can they fly.
The funny thing about an instructor rating is that it doesn't necessarily mean you are a good pilot. Most of them are very good. But I have seen a couple instructors with more money than skills, and the CFIG was just another trophy for them, and not a hard one to get if you already have a power CFI. One of these instructors went downwind in a 2-33 with a student. Did I mention 2-33's are strong, safe gliders? Even when putting them into the trees? Well, they are.
The instructor requirement for taking passengers makes no sense to me.
-- IFMU