Yeast is yeast. A "dark ale" yeast may be one that helps highlight the flavors of darker grains just as a "Burton" yeast recreates the flavor profile of yeasts used in ales from Burton-on-Trent (like Bass). You can still use them on other brews and might even like the results. As long as you are not trying to replicate a specific flavor profile (like a West Coast IPA), you can use whatever yeast you want.