Against difficult enemies, Tactical Parry doesn't matter for any job because they all use AoE attacks that you can't Parry these days. Against easier enemies, Tactical Parry doesn't really matter for any 2H job because it doesn't save you a swing. It may slightly increase your WS damage, though. Against high level enemies that don't multi-attack, even 1H jobs have a floored proc rate on Parrying. This means that DRK with their E skill is no different from NIN or COR in the proc rate during the only times that really matter, so crappy skill ranking isn't really a good reason to not want the trait.
Adding Fast Cast and/or reducing casting times for dark magic is somewhat misguided, though it would certainly help a little. Every time you cast a magic spell, there's a minimum of a 3 seconds delay in your swinging. At high Haste (which is pretty much the only way you use DRK), that's two attack rounds, half a WS, etc. They need to reduce magic casting delay before it will really be viable to cast spells while you're DDing. That's just how it is.