Haunting guise isn't made to heal alot of hp, just an item for early game.
I buy it for penetration and a little more survivabillity.
By they way, Theres lots of lifesteal items for a reason, DPS champions are dependant on lifesteal and casters was already fine before spell vamp, so dont make casters even stronger, DPS champions are almost never good in teamfights while Casters roflstomps everyone, I dont see why more spell vamp items would be needed, if it would be added it shouldnt be much more, maybe 15% on a quite expensive item but that might be gamebreaking since lots of burst damage champions (Annie, Ryze, Kassadin, Katarina, Kennen)
So no, I wouldnt say that more spell vamp is needed.