I believe that the argument here is that Mages don't normally need that level of sustain because their sheer damage output keeps them alive by virtue of deleting enemies before they have a chance to react, or making it unwise to follow them while taking constant damage. The AP champions who do build sustain usually have much shorter ranges like Akali or Vlad and flat out need that sustain. Especially considering that Vlad has to pay in health just to use his abilities, and doesn't build much in the way of health regen the way that a Zac or a Mundo would to compensate. You're supposed to not be taking damage at all with most AP champs, hiding safely behind your tank lines while casting a few spells from afar and only committing to an all in when you know you can take down anyone who's at low enough health to allow you to survive the rest of the fight.
If spell vamp were any stronger than it already was, mages would become overpowered. The sheer amount of damage they could put out would make them nearly impossible to play against.
Consider, if you will, a Veigar. His full combo deals somewhere around 3500 to 4000 damage late game against a tank who's built some MR, like say a Rumble. He sits in Rumble's ult and flamethrower, and before he dies unloads his combo, dealing enough damage to completely obliterate poor rumble. Consider that he has 30% spellvamp as well, from whatever sources he has it. Just from his ult and his Q, both of which are single target abilities and so receive full spell vamp, he's getting almost 1000 health back. Even with the reduced spell vamp from his W being AoE, it's still giving a few hundred HP back as well, letting him heal almost back up to full health, removing all the damage Rumble did to him, while allowing him to secure a kill. For the cost of an ult, which is probably already on a short enough cooldown to be ready for the next brewing teamfight, he's completely shut down another player and he's done it at no health cost whatsoever. He walks out of it like it never happened, save for a cooldown.
This can apply to pretty much any of the mage or assassin champs. If you give them an inch, most of them have enough damage potential to spread that into a mile. Brand, Annie, Vel'Koz, even the high damage AoE champions would get simply too much health back if you buffed spellvamp any farther than it is. If you notice, the higher AP items don't come with spellvamp either because champions who build it sacrifice damage potential like what flat AP mages get in order to last a bit longer through the spellvamp and hope to cast a few more abilities. You'll never see an Akali with as much AP as, say, a Vel'Koz because she's had to use one or two items to get spellvamp, which only have about 60 or 80 AP, she's probably got a Lich Bane to supplement her burst, only another 80, while Vel'Koz has been stacking up on a DFG, a Zhonya's, and a Rabadon with those same three item slots.
or 
so that means if a fight breaks out and then both teams disengage, the ADs will return with full health where the APs will not, unless they have healing in their kits.
. This gives you an absolutely insane level of healing. The build on trundle ends with something like 2.6k health at level 18. I have been at 200 and healed to full + BT shield on a single minion wave in .5 seconds with auto/Q/Hydra. It gives them an overly powerfully lifesustain option which spell vamp isn't anything close to viable for life sustain for AP champions.
is better for mord than 
if morde isn't insanely ahead of everyone else.
