Kunoichi wants to be a fiend of possession. With a succubus she would have no class abilities when the game starts and she wouldn't be finished with her 6 level class until level 18. Meanwhile a fighter with 10 outsider hit dice is just missing some feats.
Actually, I'd reduce my +6 LA by the amount of racial HD I have, giving me an effective LA of zero. ^^; Quite useful for me, since it means I could start off the game with all 6 levels in Fiend of Possession, if I wanted. It also works pretty well for certain spellcasting monsters with innate casting equal to their HD, like Glyph's Unbodied or something like a Rakshasa or Psionic Mindflayer.
Admittedly, though, it is also a pretty good boost for melee. We're about to get to the level where melee starts to really need that boost, though, so I think it's a fair tradeoff, myself.
Also, it's opening up some character concepts for me that I wouldn't have really thought of, before. For example, since most Lycanthropes gets racial HD far in excess of their LA, lycanthropy could be used to furnish our prospective Paladin with quite the epic mount...