South Africa appears to follow the "no spoiler during turns" rule as well.
On two occasions I was told by instructors not to use spoilers/air brakes while in banked turns onto base or onto finals.

Once was during a 30 degree bank to finals at 120km/h (64 knots) in an Twin Astir at about 300 feet AGL with spoilers half extended.
I deliberately flew faster to compensate for the increase in G-loading and loss of lift due to the spoilers being half out but it wasn't an acceptable answer to the instructor so I adjusted my flying style.

The "no spoilers during turns" rule makes sense if you're flying the turn too slowly and not taking the increased stall speed into account.

Example: According to the POH, a 650kg (1430lbs) Twin Astir stalls at about 90km (49 knots) with spoilers fully deployed. Add a 45 degree bank and the stall speed increases to 126 km/h (68 knots).
This is usually above the normal approach speed flown during circuits so I can conceive that a pilot who is not "ahead of the glider" could possibly fly too slowly during the turn. Throw in an uncoordinated turn and things could go wrong very quickly depending on glider type.

Maybe those grey beards have merit in their reasoning however the use of spoilers during turns closer to the ground can certainly help with altitude control during off field landings where there isn't a 1000+ foot runway available and there may be obstacles that can only be avoided with a turn to very short finals (e.g. high voltage power lines blocking the approach to a field and the field is too short to go over the top of 55 meter (180 foot) high power lines).