Dalmatica has a couple of very small boosts to a Taru BLM.
As someone mentioned, it doubles as HP down gear for Sorc. Ring and Refresh for standing setup. The only other Refresh body that can double for more than 1 use is Morrigan body - both types of Cloak and AF2 body serve really no use outside refresh unless you use AF2 body for elemental skill (please don't with 10k igqira bodies on AH). Morrigan body is pretty hard to get for most people, so I don't see how it's unreasonable to want a Dalmatica if the opportunity arises - I'm not saying Dalma is easy to get either, but certain people have better access to certain types of gear.
Also, to the person that said only Galkas and Elvaans really need a Dalma to proc the ring, just because it is possible to proc the ring w/o a Dalma doesn't make it the most efficient method. Good BLMs are constantly stressing inventory management and trying to save slots. For instance, when I converted to a Sorc. Ring user, I tried really desperately to save gear slots when converting to keep my other gear for like Dark Magic, Haste on Stun, etc.
Eventually, I was able to have every single piece of gear in my HP down set double for another use, which is the ideal situation - Zenith Head +1, Dalma, Zenith Hands +1, Penitents, and Mahatma feet (-205HP when needing -203HP). The last piece to complete this for me was getting Mitts +1, which cost a shit load pretty much on any server. I shelled out quite a bit for them, and it wasn't for the damage boost, but because converting that extra 5 HP actually saved me an entire inventory slot. That's how much a career BLM can be willing to shell out for just 1 more inventory slot. Dalma provided a similar boost, saving 1 slot.
This is all sort of nit picky though. Certain people will always go so above and beyond to get marginal gains on their favorite jobs. I think it's pretty arrogant to question someone wanting an item (by saying XXX is only useful for job Y) if they can back it up on why they want it and how it improves them, no matter how minor the improvement. When my best friend online wants to shell out 20M for an extra couple of STR for her THF, I think she's a bit crazy - probably the same way she feels when I spend the same sort of money to save 1 inventory slot.
Also, the "I have 3+ mage jobs, Dalma saves me slots between them" is a very valid argument. While some people try to deck out one job, others would rather keep all jobs are high efficiency. It's a question of trade-offs! Getting a refresh piece for 3 jobs could save you 2 slots, not from your inventory, but your overall mog house space - it's the same sort of argument as inventory management, but this time you're managing your entire mog house.