DRK is fine.

No one was exactly bitching about WAR's lack of "party utility," because WAR functioned and performed its job just fine. People were clearing os4 with WAR before the shake it off change, and that was that. No complaints, no fuss. Did the SiO buff make it easier? Of course, but it wasn't something that was going to make or break the job by a stretch. No one would be up in arms if WAR never received a party-wide shield, and no one would have quit the job if it was never given one, much less complain about it.

People like to bring up WAR having an equivalent to TBN in the form of Inner Beast, and an extra cooldown in the form of Thrill of Battle, but let's be real. IB was never desirable because it meant lost Fell Cleaves and punishment in the form of 10 seconds in defiance, and ToB was always used for IRzerk windows to buff up upheaval anyways.

However, DRK's TBN is mitigation on demand that can be used gritless. It pretty much pays for itself to break evenly. With it, you can supplement your cooldowns. Fact of the matter is that many tank busters are magical, and what better way to deal with them than use of TBN+Dark Mind? You could even go as far as treating grit like a cooldown and supplementing that alone with TBN, and that works out just fine because unlike PLD, you don't spend another GCD to go into "dps stance."

Back in HW during gordias, a PLD could not compete with the other tanks without being at least 10 ilvls ahead. In SB, this has not been the case with DRK. It can do all content at a similar ilvl as other tanks and is very well capable of dealing damage that's within the realm of the other two tanks.

This all said, DRK is performing its job just fine. It's not weak, in terms of damage nor in mitigation. Is it weaker than the other tanks in those aspects? Sure, but it's not ruining the job. It's not a reason to cry to SE that your job has been neutered simply because you lack an answer to PLD/WAR's divine veil/SiO. "Party utility," as exemplified by the earlier version of WAR, isn't exactly a strong case for what a tank needs.

If someone is struggling with DRK, maybe just maybe it's not the job but the player.