That seems to be the paper figure, I'd surprised if this actually happened regularly in practice or if even this paper strength persisted for centuries. And, again, is there any evidence for carrobalista besides a single image on a single column? If they were really all that useful and widespread, why do they not appear everywhere in descriptions of battles? I also don't believe in shooting from behind friendly lines. If you consider the depth of a cohort, the multiple lines of cohorts and the gaps between the lines, 100m is not so long. And besides, how would fire control work? The operators wouldn't be able to see what they were aiming at: what happens if you have your own skirimishers in front, or the first line of cohorts pushes forwards more than expected, or your cavalry is flanking the enemy? It's going to take some very robust evidence to deal those major hurdles.
Still, even with these idealised figures, this makes a scorpion about as deadly as, say, a longbowman. Scorpions would have had better armour penetration probably, but I'm not sure how important this is if you can go through most armour already. So the scorpion contingent of a legion would be roughly the same as having 60 medieval longbowman teleported in, but slower to move around and requiring multiple men to use. Useful in some situations, but a drop in the ocean compared to some 4500 heavy infantry.
There are only a few situations where I can see them being really useful. As stated before, in sieges, attacking or defending; this should be obvious. The other would be area denial. Say you want to cross a river, but archers on the other side are rudely shooting at you as you attempt to cross. You put your own scorpion/ballista on your side of the river, outrange the enemy archers, and take pot shots at them until they see the wisdom of taking a few steps back, thereby letting your vanguard ford the river more safely. A variant of this is to force the enemy out of a defensive position. If they are formed up on top of a hill and you want to dislodge them, a frontal assault would be difficult. But you can deploy your artillery in the front rank and start shooting. They can then respond in three different ways. Firstly is to just stand there, and slowly die (this is the worst option); secondly they can retreat from their strong position, thereby granting you your objective. Or, thirdly and most likely, they'll have to leave the hill and close with you, at which point your infantry steps in front of the scorpions and fights as normal. Note that in these cases the scorpions don't have to fire a single shot to be effective, by giving the Romans a wider set of tactical options, they force the enemy to react.
But these are relatively specialised situations. If they were very helpful in standard pitched battles/meeting engagements, then where are the historical texts describing their use in every battle? It's all well and good to theorise, but if there is scant evidence that ballista/scorpions were routinely used to great effect, then they probably weren't routinely used to great effect.
