The difference is the Ardent Defender would just have a skill called 969.
Anyways. The necromancer idea I really like. Originally when the thread was brought up I was typing my post without reading the responses, then I read them in the middle of my response and hit cancel when seeing another post. basically born off the idea that a new "hero" class should be a caster, and if it can heal the better. Basically the necromancer feeding heals to the party by sacrificing their minions and drain type effects. Also abilities that link the health pools of two members (Ripe for abuse!).
I was having problems trying to somehow balance a class that can DPS and heal at the same time in my head. They certainly can't do damage near the capacity of other DPS in their "healing" spec, or healing as well as other classes while getting free DPS. Maybe having some sort of "swing time" between their maximums.
I just like minions (such desires are culminated in games like Overlord), and one of my favorite abilities in WoW is the druid's treant skill. I think it's just born of my desire to have my own lackeys.
I actually tossed around the idea of a healing class that primarily healed through their own health pool. I didn't associate it with necromancers (though that could certainly work) I even had a very similar idea to the link, except I think I was thinking of having the necromancer link their health pool to someone else.
Such a class could regen their resource (hp) through dots and attacks which leeched health from the enemy (use for mana). Their damage could be weak, the heals would probably have to be multiplied, but that might go along the lines of what you were thinking.
Of course, if it were to follow the DK hero class model each tree would need to be viable for healing and DPS.
So you could have one tree as a self-sacrifical healer, one where they might use minions to heal, and then I don't know what the third would be. And, of course, all of those trees would be plenty capable as DPS.