Carbuncle.Maletaru said: 
»If you started now, you'd have 10 REMA just from running ambuscade by 2029. Unfortunately, you probably still won't have a reason to make a prime ><;;
I get this is kind of tongue-in-cheek but...there's no more "reason" to make a Caladbolg than there is "reason" to make a Helheim. You make them because they do a lot of damage.
I find it really strange that in an MMO it's perfectly normal to chase +1 light elemental resistance for your THF idle set on the off chance you're clipped by a stray Diffusion Ray while kiting, but we draw the line at getting another 10% DPS on DRK (made up numbers) because "you don't need it."
I don't think it's normal to chase idle gear. I think it's normal to learn how to kite so you don't have to chase idle gear. I would only suggest to someone that they chase gear that's critical to their role/function.
If a player wants +1 Light Elemental resist to go from 0 to 1/8 (12.5% chance of a resist), I say go for it. But to your point, if someone is sacrificing DT in their idle set or Haste in their TP set, their effort is wasted because they're making a premature optimization. (This happens all the time because it's easy for some players to get overwhelmed by how many variables are at play in a game like FFXI).
I would just make sure their optimizations are appropriate for the situation being discussed. Otherwise, trade-offs and risk assessments exist. SE implemented FAFO before Dark Souls existed as a genre. 
So, while I certainly appreciate your argument, I think all that is left to do in this game's long life, is to help your friends collect pieces of gear. Yes, CHASING DPS has been extremely common and is the empirical justification for EVERYTHING in FFXI....and you know what? That *** sucks!
Let's say SE created elemental instances of Limbus/Apollyon (Fire, Ice, Earth, Thunder etc). BUT ...Instead of Elemental Resistance affecting your defensive capabilities, you are able to deal increased damage based on your Elemental+ gear/spells. If you're in a Fire-aligned zone, wear more Fire+ to deal more Damage. If you're in an Earth-aligned zone, wear more Earth+ to deal more Damage. There's a lot of ways SE can do this.
But on the topic of THF wanting +1 Light Elemental resistance, I say go for it, if they're doing fine in their other responsibilities. This game is meant to be entertainment, do what you enjoy. (I think mine is theory-crafting).