I've always thought slings are under-rated. If you ask me, there is something much more frightening about having a rock flung at you at supersonic speed than an arrow - something hideous and terrifying about the thought of a hefty chunk of stone hitting you FULL IN THE FACE and shattering teeth and bone.
From Charles C. Mann's 1491:
Although Andean troops carried bows, javelins, maces and clubs, their most fearsome weapon, the sling, was made out of cloth. A sling is a woven pouch attached to two strings. The slinger puts a stone or slug in the pouch, picks up the strings by the free ends, spins them around a few times, and releases one of the strings at the proper moment. Expert users could hurl a stone, the Spanish adventurer Alonso Enriquez de Guzman wrote, "with such force that it will kill a horse...I have seen a stone, thus hurled from a sling, break a sword in two pieces when it was held in a man's hand at a distance of thirty paces". (Experimenting with a five foot long, Andean-style sling and an egg-sized rock from my garden, I was able, according to my rough calculation, to throw the stone at more than one hundred miles per hour. My aim was terrible though.)
In a frightening innovation, the Inka heated stones in camp fires until they were red hot, wrapped them in pitch-soaked cotton, and hurled them at their targets. The cotton caught fire in mid-air. In a sudden onslaught the sky would rain burning missiles. During a counter attack in May 1536 an Inka army used these missiles to burn Spanish-occupied Qosqo to the ground. Unable to step outside, the conquistadors cowered in shelters beneath a relentless, weeks-long barrage of flaming stone...I'm toying with bumping sling damage up to d6 for my games, or allowing two attacks at d4 damage to reflect the speed with which they can be fired.
They're also one of those weapons that no sensible adventurer should be without. The ideal D&D fighter, in my view, uses a hafted weapon - a spear or trident - and sling combination, probably also with a bola or net if the DM allows it, and a shield, of course. The spear or trident doubles up as a pokey-thing for poking stuff, which is one of the most crucial adventuring tasks. And the sling provides essentially infinite ranged attacks: if for some ungodly reason you can't find rocks to fire, you fling copper coins instead. If push comes to shove, you can also use it is a club by putting a big rock in the pouch and whipping your opponents.
Finally, there are records of troops in resistance movements in World War II using slings to fling molotov cocktails. No problems with flasks not breaking there, I think.