The fact that gigamilions of launches take place successfully only proves that the release mechanism is an excellent design & a weak link is not required because pilots are not trying to pull the wings off.
You are right that release hook design is excellent. It is not intended to release due to overload though, so the weak link is still an essential component. As Blind Pew pointed out, a thermal (or a gust for that matter) can suddenly increase the lift from the glider and the tension in the cable, before the pilot or winch driver can react to keep the tension in limits.
With winches typical putting 300hp+ into the cable drum these days, and their engines typically 30% efficient, there is 700hp of heat heading downwind to create, or at least accelerate, a thermal.