What if we had Jungle Quints for bonus monster damage?
5% additional monster damage per quint, for a maximum of 15% with 3 quints.
For comparison, most junglers start with less than 70 ad and 15% bonus damage would be worth about 10.5 ad discounting base damage (since the vast majority of the damage is just raw auto attacks)
Also, most junglers don't have any attack speed from runes or masteries, and 15% bonus damage is worth 15% bonus attack speed in that case, and you get precisely 15% bonus attack speed from attack speed quints.
So why take the jungle specific runes if you can get the same damage to monsters at level 1 with other quints? Because they scale harder for specifically damaging monsters. No other reason.
These runes are for champions that have a really hard ass time clearing the jungle even AFTER their first back.