This came up last night in our gaming session and I think a new alternate rule (or maybe a duplicate of some homebrew rule somewhere) was proposed that could be of great utility.
The situation was this: a small island-shrine dedicated to Meri, Lady of the Waters, had been overrun by ogres and their enslaved cyclops. The Hounds, deciding that since they intended to sail across the Cloud Sea to the farthest north, went to curry some favor with the Pale Lady and get her blessings (perhaps to counteract the ire they had raised from her husband, Vodei, the Old Man of the Deeps). It helped, to be sure, that one of the Hounds is a cleric of the sea-goddess.
Learning of the possibility of giants, however, the Hounds decided they needed to come in hot. Rather than taking a small ship or the little slip used by the Merianites for transport they went to the Harbor of Stars in Tyrma. The worsening fall weather had grounded a number of ships for the season and they hit upon an evil looking vessel out of Blackport: the Black Wind, a crew of monstrous beasts manning her, and her captain Tariq.
Well, off they went to the island and, lo and behold, they alerted the ogres by their very presence. Soon enough the chained cyclops was out and throwing rocks and...
Oh wait, how do grenade-like missiles work? You make a regular attack roll and if you miss, it scatters? That doesn't make very much sense...
By that logic, a man in plate armor is harder to hit with a 10 ton boulder than a spry little fellow. Forget the boulder, how about a flask of burning oil? The very purpose of many grenade-like weapons is to overcome naturally tough defenses (armor, say) since you don't actually have to pierce anything to hit someone with them.
So here is the alternative rule: the attack is made against AC 10 (the ground) except its made as though untrained (-2 for warriors, etc.) and the -2/-4 distance rules are factored in as well for medium/long distance. NOW we're cookin with gas! And if the projectile is particularly large, why not allow a saving through vs. breath weapon for everyone who isn't the direct target but is within 5' to avoid half damage?
Sounds a lot better to me.