I've been thinking about the idea of using a bonesinger along with 5 wraithguard/blades in a serpent, and to fill up the serpent why not bring a spiritseer as well.
Wraithguard will pop out within smite range, so both psykers would be in smite range, healing the wraithguard/blades is only useful 2/3 times or less, and even then only for a maximum of 2 wounds.
Healing the serpent is not valuable enough either, but if you can make the serpent filled with wraiths scary enough, someone is going to attack it, maybe you should take the most aggressive weapons on it, to make sure it's a big threat, this way the serpent is going to be damaged really fast. Even if the serpent is destroyed it might explode and damage the passangers, which would then be a target for the healing.
Actually choosing to going second is not bad for eldar in general, but with bonesingers, it gets even better, because they might be able to do their thing from the start.
While the bonesinger is slightly overpriced when not allowed to both smite and heal, it's not by much, and if a list is built around them, say mechanized Iyanden spirithost, they WILL make their points back.
Taking say 3 bonesingers along with 3 serpents full of wraithguard/blades, would allow those bonesingers to jump ship too, or even support a knight or lord nearby, when they are otherwise idle.