I think the main reasons I have remained in support of Dark/Electric over Dark/Grass are the following:
1)Competition with Venusaur as a Grass type is a possibility
2)Inferior coverage against Rain-mons, namely Tentacruel and Gyarados
3)In the case that CAP5 gets Volt Switch, it won't have STAB anymore. This move is kinda good for a pivot, particularly one against water types.
4)Becomes weak to Hurricane
5)Becomes weak to Ice Beam
6)Stops resisting Bullet Punch (and can't touch Scizor with a STAB move at all either)
Dark/Electric is still superior to the late addition, Dark/Grass.