Life steal vs Spell vamp

Spell vamp is a defensive stat which converts a percentage of the damage from either champion abilities or item actives into health. It only works for active champion abilities that do not modify basic attacks or apply on-hit effects (including true damage abilities) [1].

Life steal is an offensive stat that restores health to the wielder for a percentage of all on-hit physical damage they deal, except for Corki's Basic attacks, which also applies life steal on magic damage. Basic attacks, abilities that apply on-hit effects, and abilities that modify basic attacks all benefit from lifesteal.[1]

so the reason Roit cant push the spell vamp stat is in short you can get way more out of Life steal because AA speed champs will out scale any mage or AP champ that has Spell vamp. since spell vamp dose not apply to AA on mages or AP champs.

this is the why. Spell vamp just needs to be reworked to give more life to make up for the LONG CD ability's like W vs AA like . Yi doesn't have to use any Ability's to stay alive just AA . for Spell vamp to work for AP champs they would need to work off the AA as well .

thats the only way to make Spell vamp worthy stat over mana or CDR or more AP. your thoughts?