As things are now, i do kind of see them as useless, but I also kind of figured they weren't entirely finished either. the devs have a tendency of introducing new stuff for later but not actually fully polishing or revealing it until a bit later to. I could see a lot of potential use for that ballista though, that just isn't realized fully yet. Just as an example, imagine they do rework oceans and make that it's own biome challenge, potentially with it's own boss, I could totally see the potential of a new boat, that also allows ballista placement for ocean related challenges like that (presumably a war ship would be a later game thing). No idea if that is a potential future, but it's the kind of potential I can see for ballista if it is one of those "well, we havent actually finished working on this but its still fun to tinker with" additions the devs do.

As it is right this moment though, i just don't see much point to it. It's resource intensive to possibly quite literally shoot myself in the foot with. (Okay not "resource intensive" by the standards of resources you have at the time, but i usually dont spend resources like black metal on hurting myself either). Maybe I'm just not giving it a fair chance, but unless it is mistlands where the ballista might "see" something in the mist before i do I haven't found a lot of use for them that I don't prefer to just handle personally.