User blog comment:Willbachbakal/(Item Concept) Spell-Based Sustain/@comment-7709681-20141106142423/@comment-1330314-20141106150147

The idea here is that the sustained healing offered by provides more healing over time than, but the latter offers a ton of burst healing, which WotA can't provide. I may have implemented WotA's healing a bit too generously, though, and so if the numbers don't work out the healing could always be capped differently (i.e. healing would be capped every second or half-second, or per spell cast), but if you're a burst caster you're going to be healing for a ton of health in a very short amount of time, which a sustained damage dealer wouldn't be able to do.

I also made a deliberate decision to avoid having spell vamp work off of anything other than magic damage, mainly to avoid bizarre edge cases in which bruisers might pick up the item for spell-based healing. True damage is a rare instance of damage that would only really affect and  in this case, and even then the way each item provides healing means you're not dependent on your true damage to make full use of the spell vamp.