Again, I doubt that legionaries were EXPERT slingers, but trained enough to toss stones down range en masse? Hell, I can do that with a sling. So potentially used en masse in sieges. And, again, legionaries had lots of down time where knowing how to use a sling would be great in supplementing rations. So out of five thousand men, having a couple hundred who are expert enough to skirmish would probably be a thing, and using them as light, skirmishing antessori (sp?) would be a possibility.
Also, again, they’d be very much like musket armed skirmishers: tossing rocks downrange at massed bodies of troops, 4-16 deep…? You don’t need to be Balearic quality for that.
We get slapped around, but we have a good time!