Currently reading and using the Iyanden codex, there is one nice bit of fluff where the Iyanden Craftworld is attacked by Orks. During the battle on the craftworld a web way gate opens up and the Dark Eldar rush to their cousins aid. No explanation given, but they do finish of the Orks and take any Ork survivors as slaves. Just as quickly as they appear, their gone again. My thoughts are that the Dark Eldar although have obvious similarities to Eldar, do fight very differently and have a completely different agenda. They deserve to be treated as a different faction in this game as much as they are within the TT game.