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.
Actually the only real hindrance is the fact that you get the spells later/mp(race dependant) and you have no access to tier 3 nukes. DRK have a B+ in Elemental skill, matching SCH and is stronger than RDM's C+. But that doesnt mean they nuke in experience points parties, and is more of a solo endeavor. In parties you usually use Absorb spells to boost your melee than actually use them to do direct damage.

However in relation to Absorb spells, they were lowered in cast time to two seconds, and therefore is much more useful provided you get some form of Refresh(mana regen) and the Parade Gorget/Vampire Cloak helps you alot with that during downtime. You can i think get up to +9 or 12 STR temporarily and thats alot considering what people pay for those kinds of gears. Not to mention Absorb-Acc.

From level 20-30, /DRG is a good option, since DRG receives the passive ability Accuracy Boost, which grants +10 acc.
/RNG is the one that gives you a +10Acc. /DRG gives an Attack Bonus that you already get at lvl 10 and it does not stack. Only thing worthwhile would be a Jump. Although Ouch! is right...you probably wont be using /BLM anytime soon with the exception of Warp.

With Hasso you receive the same bonus as a SAM does at lvl 37 if you are at 75DRK. So a +5 STR, Acc, and Haste.(very good)

Using a SAM sub though you can let loose a little better after level 70 with Dread Spikes, and Seigan + Third Eye combo. Before that, its a little scary to use WAR sub with reckless abandon. I personally find DRK to be very good in endurance if they can use Aspir, Refresh gear, and absorb spells often. So in the end, i think they rawk. But dont expect awesomeness the minute you get it...its more of a work hard, get awesome later kind of job imo. But every step on its way is pretty exciting with spells and abilities.