The majority of gliders are more stall and spin resistant at medium to steeper banks than at shallower bank angles. (This is aerodynamically different than most airplanes). A continuous 30 degree bank from downwind to final exposes a pilot to a longer period of stall/spin-at-low-altitude risk than two brief periods of stall/spin resistant steeper banked turns. (Or 3 turns in the clipped base pattern).

Turning flight presents a more dynamic visual picture then straight flight. Most normally equipped humans are better at assessing and reacting to the changing energy state of the glider (relative to landing area & speed) as well as detecting conflicting traffic and other hazards during wings level straight flight than during turning flight. This may be due to the less dynamic visual presentation in straight flight. This is especially true while under stress.

While it is true that many of you normally performing well trained pilots can safely fly all kinds of approaches, common sense suggests training and establishing flying habits that are more likely to result in safe outcomes when normal conditions and normal performance deteriorates.

The fact that we are still debating these things in 2016 makes me want to beat my head against the canopy.