Thread necro to rival the Great Sleep of the Necrons, but there's a cool link I've just thought of...
I love the idea that 'empathic vampirism' is a trait that all Eldar share. It makes a lot of sense from a fluff standpoint, given that Eldar are able to move between the various Eldar cultures.
It also adds a little more of the 'alien' to Eldar as a whole. They're completely, utterly different to us humans. The literal ability to drink souls to sustain them, whether Commorite or Craftworlder, adds more depth to this. An entire race of empathic vampires. Grimdark
Combine that with this picture:
http://wh40k.lexicanum.com/wiki/File:Runes.jpg
Bottom-right is a description of the rune-concept 'soul drinker'.
It's a completely distinct rune from the one for Slaanesh, and is one of the most hideous rune-concepts in the Eldar language.
Imaginative headfluff time: what if there were Eldar pre-Fall that subsisted by drinking other Eldar's souls? Sounds like it would be a heinous crime to most pre-Fall Eldar (and current Craftworlders) based on the description of the rune above. Especially considering that in those days Eldar were able to reincarnate. It wouldn't be just equal to killing someone, but would also deprive them of all the future lives they could have had as well. More grimdark
Post-fall, the Dark Eldar discover that this terrible ancient practice offers them protection from the predation of She-Who-Thirsts, and adopt it through necessity.