Let us look at the combat support role/combat multiplier role of slingers in ancient armies. My thoughts on this subject are that the slinger troops of an ancient army existed to create casualties in any way they could, and thereby weaken an attacking or defending enemy force. In either situation, slingers could create a very hostile environment that was certain to "soften up" enemy formations. I think too much emphasis today is being placed on the effectiveness of slingers to make "kills" rather than placing troops out of action by merely wounding them. My personal theory is slingers would do their best to wreak havoc on opposing troops as long as they could before the opposing forces came to sword stroke/spear thrust. I do not wish to minimize the effectiveness of the sling and the men that used them as viable instruments of warfare, but only want to approach their employment from a slightly different angle. There is no doubt in my mind that slingers contributed materially to the battles we read about, but they seem to be forgotten by many historians.

most armies most of the time used ranged troops,melee troops and cavalry. So if there were slingers in a army,most probably they started bombarding the other army as soon as they were in range until they got too close to sling at them. During this time they probably got some kills,but most probably the majority of hits were injuries of varying severity. The ratio of kills to injuries varied according to the targeted army's defensive equipment(helmets,shields,armor),the more armoured,the less kills and wounds.
Obviously ...
Also the bombarded army would probably have a very hard time charging while stones\lead is raining down on them. Running with the shield covering you from above ain"t the fastest charge,and you might get an arrow to the belly while doing so. This combo would be highly effective as i can imagine..high arcing slinging at about 100m meanwhile the archers are loosing arrows on a flatter trajectory.
Cavalry would also have a hard time charging in a hail of stones. Horses would get probably jumpy\scared at the noise of whirring stones,but that's just my imagination..

This is exactly what I'm driving at. Be it an offensive or defensive situation, the slingers had the capability to begin engaging the opposition at long range with a view to sow confusion and create havoc. With this in mind, it is likely their "fires" could also cause the opposition to modify it's initial scheme of maneuver and spoil an otherwise well thought out plan.