There are a few problems with this. While the scythe does indeed have a lot of accuracy on it, it doesn't change the fact that scythe WS's in general are very lackluster, whereas dagger has Rudra's Storm. +70 to the crappiest weapon in the game doesn't mean much. Secondly, most DRK's would rather use a GS, weapons that don't have that kind of accuracy boost available. You could argue the real problem is that grips in general just suck compared to wielding a second weapon. I think the most amount of accuracy on a grip is like what, 10? That's pathetic. Finally, 2H jobs have to sacrifice a lot more than dual wield jobs when swapping to accuracy gear. They often lose their DA, TA, critical hit, and most importantly, their X-hit setups. I realize dual wielders also suffer from this, but it affects the 2handers on a greater scale.
Comparing the best versus the best is an ineffective way to measure. While it's easier to get a Cronus with acc+70 than a capped tami sari, it also means less because DRK has lower natural accuracy than thief,and especially dancer, and don't even get me started on BLU. Also keep in mind, not everybody has access to the best equipment, which is a lot of what this thread is about. And like previously stated, scythe WS's are very unimpressive compared to sword and dagger. There's just no place for a scythe wielding DRK of moderate equipment in these groups. Aside from SAM, 2H jobs are rarely a welcome sight in high-end content.