I don't see them ever touching Caster LB3 if only because it's now ingrained into The Weapon's Refrain (Ultimate)'s raid; there's a section in the fight where the caster has to LB3 to destroy the targets around the arena or the party will wipe. That said, I don't entirely disagree that Ranged LB3 and Caster LB3 do sort of serve the same purpose. I might get some heat, but perhaps the Ranged LB3 should be changed; either shifted back to Healer LBs as they were in 2.x.

A Limit Break for a role is the ultimate move of that role, fueled by the power of the entire party's spirit. For melee, it's a massive single target burst; for casters, you're letting loose an apocalyptic level spell onto the arena. For healers, you're defying time and death itself, rejuvenating all of your companions; for tanks, you're creating a nigh-impenetrable wall/force shield around the party. In this sense, it makes sense Rangers would have a powerful directional attack piercing all defenses, but another more important aspect of Rangers in this game is the party support.

A Limit Break based around their party support could manifest a few ways. As mentioned earlier, copying the Healer LB could be a thing; it lets a Ranger possibly save a wipe when the healers are dead. But many people disliked having that on Rangers (in 2.x, I also disliked it), and it apes Healer's LB identity. Another way Ranger LB could manifest is Resource management, since that's a game of a Ranger: refresh and tactician. But that would be incredibly weak and used even less than Ranger LB is used now.

Maybe it could manifest, instead, as a statboost for the party--call me crazy, but maybe let Ranger DPS have a (watered down) Ungarmax-style Limit Break. Back when that bug was a thing, people loved using it (maybe yes because it was so potent). But think about it: a Ranger LB3 that's Ungarmax (again, basically, not the real Ungarmax...though maybe yeah for LB3, real Ungarmax), and has half the animation lock of LB3.

The drawback to Melee LB3 is that there is a cast time and the Melee is putting themselves out in harm's way. Same-ish with Caster LB3 but I don't generally see CLB3 used very much either. I digress! So the advantage to Rangermax LB3 would be that it doesn't put a player in as dangerous a spot: the tradeoff is that in the long run, RLB3 does less overall damage than MLB3.

Another drawback is Rangermax would become the gold standard to pad FFLogs. In which case FFLogs would have to censor Rangermax clears out like they did Ungarmax, which would then make Rangermax extinct (or they could do nothing...and we'd never see anything but Rangermax from DPS). I apologize, I'm using your post to ramble off random ideas.