We've also been curious as to why we NEVER break one of the built-in weak links, carefully chosen to be the appropriate ones for the weights of the gliders we tow, while on several occasions we've broken the cable just as it enters the glider-end slug. (I think the cable flexes more at that point, possibly weakening it over time.)

Yup - we had exactly the same experience - the rope would break in front of the Tost weak link unit or someplace else (resulting in loss of the weak link). Honestly, I can't remember ever seeing a Tost weak link break as it is supposed to.
ROY