I've played a tarutaru Dark Knight until level 40. I'm going to be perfectly blunt, even though I had the highest INT and MP offered by any race, I didn't do much nuking at all. I used drain and aspir (dark magic) frequently, but other than that, not so much. At the high 30's, DRK starts obtaining the Absorb spells. These spells steal points from a monster's stats and add it to you own. With a high MP pool and INT, they're probably decently useful. Absorb-STR will make the monster hit softer and you hit harder. Absorb-VIT will rob the monster of defense, making you and everyone in your party do more damage as well. There's also stun, which was a lot more useful before BLU. DRK gets stun before BLM, but now BLU gets an ability with stun much, much earlier.

Unfortunately, most DRKs don't possess the MP pool to use these regularly. In fact, the only spells most DRKs will use are drain, aspir, and Absorb-TP. If timing permits, you can also use elemental nukes for magic bursts off of skillchains. Conventional nuking is not possible. DRK receives the spells later than RDM, BLM, and SCH, and its elemental magic skill is much lower, too. Nukes are inefficient compared to the damage DRKs can do by melee, and FFXI is all about efficiency.

That said, let's discuss subs. You'll never use /BLM in an exp. situation. I'm not sure whether or not it has uses end-game (my guess is not likely), but it's not worth it in a party. In parties, /WAR, /THF, and /SAM are likely your bread and butter. From level 20-30, /DRG is a good option, since DRG receives the passive ability Accuracy Boost, which grants +10 acc. At 30, though, with /WAR you'll receive Berserk (increases attack, decreases defense) and with THF you'll get Sneak Attack (critical hit when attacking from behind).

Ever since an update last October (or around then), two-hand weapons have received significant boosts. Conventionally, 2 STR = 1 attack and 2 DEX = 1 acc. Two-handed weapons, the relationship has been altered. Both operate on a 4:3 ratio now, making two-handed weapons hit harder than before. At level 25, Samurai learns Hasso, which grants bonuses to STR, accuracy, and haste when using two-handed weapons while imposing penalties on casting time and recasting time. At level 50, DRK will receive this with a /SAM sub. I'm not sure accuracy and haste are are included when it's used from a sub, though.

At level 60, you will also obtain Trick Attack from /THF, which allows you to perform some pretty massive spike damage through weapon skills by combining SATA.

/NIN is used occasionally, more often for the addition of utsusemi: ichi and utsusemi: ni than dual-wielding.

Anyway, as you can see from all my examples regarding possible subs, none of them really capitalize on DRK's ability to use magic. That's my point, though, you won't be nuking. Aside from an occasional magic burst, your spell repertoire will likely be limited to drain, aspir, and the absorb- spells (almost exclusively absorb-TP, in most cases). Race doesn't make a significant enough impact to change this (even a tarutaru DRK can't nuke effectively). In this case specifically, can does not mean will. You can nuke, but you almost always won't.

That said, if the idea of being a good melee damage dealer appeals to you, go for it. I've just rejoined myself, so I'm not so sure of how much of the bias against DRKs still exists. I had fun playing the job. I stopped playing because at level 40, the accuracy obsession meant I'd be sacrificing all my STR+ gear for accuracy+, which, as a tarutaru, really hurts my DPS. Sure, I hit slightly more often, but I hit for peanuts. With the changes to two-hand weapons, though, accuracy gear has become a lot less of a must, allowing for more use of DEX (which aside from increasing accuracy will also increase your critical hit rate) and STR.