First thought I had when reading the "factionless"-part for the first time was that it is a safeguard because the 6th Ed. Codizies don't have factions written into their profiles yet ... but on the other hand, Harlequins were removed from the Dark Eldar Codex to possibly be dataslat'ed like Imperial Assasins, the might come out be factionless and legal to include into both Eldar and Dark Eldar detachments without being assigned to the Dark Eldar or Eldar faction (so they don't get be an allied detachment like Imperial Assasins)