I'm not sure that the Polish Lance was really "intended" to break. I think it's more that the winged hussars' heyday was in the 17th century, when armor was becoming much less common and it would have been more import to have a Lance long and light enough to compete with infantry pikes than to worry too much about strength and power.True.
IMHO, it exactly why the Polish lances had a particular design and they were breaking extremely easier than "classical" lances. (lance breaking should be the "little give somewhere")
They weren't exactly designed (nor used) as "classical" lances.
Actually, the leather strap wasn't there for the purpose of of increasing the mass. The role was to liberate hussar's right hand of the weight of the lance (as it was extremely long, the gravity point was far in front of the arm) and give the hussar the ability to easily aim the point of the lance (as the lance had a practically fixed/steady point in the rear).
Stories say that it wasn't uncommon that hussars impaled not one but two infantrymen in one trust.
Edit: I've forgotten to precise: the Polish lance was a light lance, not a heavy one and a very rigid one.
I'm a bit unclear on how often the leather cup was actually used by lancers in the 1600s. Sir John Smythe gained experience fighting around Hungary in the 1560s or 70s and does mention that some Eastern horsemen would use a leather cup attached to the saddle, but he only associates it with light cavalry, which suggests perhaps to me that maybe it began as a way to brace the Lance in lieu of a solid breastplate, or as a way to put a bit more power behind their light lances or spears if they ever needed to charge. For the most part though the lances/spears used by light cavalry in eastern europe at the time were supposed to more flexible weapons and good at thrusting, turning or spearfighting rather than just couched charges. The way Smythe explained it the stradiots/hussars/turkish light cavalry/etc. compared to western heavy cavalry would prefer to fight in a much looser formation in sort of a half-moon. If a dense squadron of enemy heavy cavalry tried to charge into the middle then the light horsemen would split in two to let them pass through the middle and then ride around the flanks and rear of the enemy, stabbing at and wounding the unarmored parts of their horses.
The Polish hussars towards the start of the 17th century though became something very different to most hussars of the 16th century and kind of hard to compare the two.