You've got some of the numbers wrong. When you beat a boss area, there's a 50% chance the boss will drop a SILVER chest, and if one does, there is a secondary 1% chance that chest will become a jeweled chest. So there's effectively a 0.5% chance for a boss to yield a jeweled chest. The talent that doubles jeweled chest rates simply brings that "1%" up to "2%", which means it's still a 50% chance for a silver chest, then a 2% chance that chest will be jeweled, meaning jeweled chests have a 1% chance overall with the talent maxed.

Assuming you have Ana'a legendary clover, she would boost that silver 50% by 30% additively, making each boss have an 80% chance at dropping a silver chest and STILL a 2% chance of that silver chest being jeweled, meaning a 1.6% chance of dropping a jeweled chest, overall.

So if you fight 300 bosses under Ana's legendary-buffed effect, you'd be getting 300 x .8 = 240 silver chests, on average, with 2% of those silvers becoming jeweled, so 240 x .02 = 4.8, meaning about 235 silver chests and 4 or 5 jeweled chests.

It is incredibly useful, not useless. It DOUBLES your jeweled chest find. Normally the rate is a mere 0.5%, so if you think of it as "Just adding 0.5% more" then yeah, it sounds small, but jeweled chests are rare regardless. It's going to be a small chance no matter what you do. DOUBLING your chances of finding them is enormously useful. It is one of the best new talents of the bunch because the "endgame" is all about farming materials.