I am under the impression that Dark DME is just pale DME mixed with other roasted grains. The problem wiht this is that you do not get any control of the ratios of roasted grains, etc. if you use Pale DME, you get full control by adding all of the extras yourself. If you use Dark, you still have the majority of control, but there will be some flavors you cannot account for, because they were inherent in the DME. And all different manufacturers use different grains and ratios, so no two are the same.
If you use a Dark DME from a specific company and love the way it tastes, then keep using it, obviously their recipe works with your palate and brew style. But if you want more control over all of the flavor sin your beer, use DME that is as Pale as possible, just 2-row.
EDIT" D'oh! Too slow!