For the hundredth time, Spellvamp and Lifesteal are not alike as effects.
Lifesteal, as a stat, is sustain that comes from sustained damage, and thus has the same nature as the damage it is originating from. Making use of Lifesteal, in the general case, requires you to be close to the fight and keep close to the fight, always rendering you open for a counter attack. Another trait of Lifesteal is that its effectiveness is generally dependent on having a high damage output, restricting its use on resistance-heavy builds. Thus, it is mainly a defensive tool against sustained damage threats for sustained damage champions - someone with heavy Lifesteal is either full health or dead.
Spellvamp, by contrast, does not have the same restrictions - all abilities that would benefit from it have base damages, and the burst-based nature of abilities makes the healing also bursty. Also, scaling heavily with base damages allows its use with resistance-heavy builds - which has been abused before, just not on Summoner's Rift. Why do you think that the original Lord Van Damm's Pillager was removed from Twisted Treeline? A further point of scaling with base damages is that it enables viably purchasing of Spellvamp early, unlike Lifesteal.
Finally, Spellvamp - as in, sustain base on ability damage - is still in the game, in form of Death's Dance's healing for physical damage and Gunblade's healing for all damage. Riot merely removed it as an option for cases where it would be overly restrictive for champion balance.
or 
and 
they can heal 300+ per auto late
with Warlords and 
