Well, the issue with dark (and amber) LME and DME is in creating recipes.
What's in the dark LME? Well, no one really knows and the manufacturer isn't saying. Certainly some crystal malt. But how much?
So, how do you know how much to use in your recipe if you don't know how much you already have? You could end up with a cloyingly sweet dark colored beer. Or not. It's hard to say.
With light LME or DME, YOU control what's in the beer. Just like with all-grain brewing. We don't have "dark two-row" to make the base grain. We have pale two-row, and then we add crystal malt, roast barley, etc, to make up the recipe.
I think people should do the same with extract brewing. Make up the recipe just as you would for an all-grain batch. Just use extract for the base malt. Then you know what you have!
To make any AG a partial mash beer (or extract beer, depending on ingredients), simply sub .6 pounds light DME for two-row base grain. Keep the crystal malt and other specialty grains the same. It works!