Having almost 50% spellvamp is very nice. Since you're nuking so much as a mage, might as well use this mechanic as another way to survive long in teamfights. (Works esp with damage over times such as malzahar's maleficvisions. Put on other lanes and laugh as someone wonders why you're healing so damn much during a battle)
40%*
Then you throw in the fact that since your AP is lower, you're killing slower, thus draining less health.
And then throw in the fact that spell vamp is cut in half for AoEs.
Okay, so lets say you threw some AoE into a group of 4 enemies.
It does roughly a total of 1500 damage let's pretend.
So you healed 300 damage. Now you have to wait 8 or so seconds to use that spell again.
Any champion, even a tank, is more than capable of dealing 300 damage in less than 1 second.
So yeah, skip out on that 40% spell vamp and get yourself a nice Rod of Ages or RCC for some health, which will definitely do you much better in a teamfight, considering AP, health, mana/slow. How could you go wrong with those items? Especially at a lower cost! Let's not forget about hourglass, getting focused? Go invincible for 2 seconds (even if you have a persisting effect you threw on someone/yourself!) and let your team take the hits, then clean up after you come out of your shell with amazing AP/armor stats! How could any mage ever go wrong with an item that gives them magic power, and resistance to magic power, while also giving off an aura to make them more vulnerable to magic? That's right, grab Abyssal Scepter at only 2650 gold! Why not throw another shot on your burst? Grab deathfire grasp or lich bane! Too much enemy magic resist? There's a void staff for that!