I personally don't think so because it's giving you the advantage of range so why not make it something challenging yet rewarding?
A lot of the time range is more of a default because you're already at a disadvantage. ie They have tier 2 weapons and you can't get close to them at all. It's already pretty slow to launch rocks and, if you're already on fire, hopeless to try to incapacitate anyone with them except the least experienced. 1.5x - 2x normal attack damage is what I would have in mind. I don't really find them rewarding so much as 'omg guess I'm throwin' rocks and hoping for the best!'
The only time I find them particularly useful is when the survivor is at medium range and I can try to slow him down. And this only works if he's out of the immediate range of his party.
slow (-), hard to aim (-), range (+)
=
not rewarding
OR
perhaps they could allow the rocks to do a little AOE because I haven't seen this so I'm not sure they can hit more than a single target.