Just had the "No points will be given because you are in Low priotity" message appear even though I'm not in low priority and the neither were the other players. Weird.
Well aside from all that healing.
And that's the one ability that could qualify him as a support. Even then I would say you need farm to have the mana pool and survivability required to to spam it throughout the course of a team fight. Also, it does more damage than heal so it's almost more of an offensive ability.
Necro's other abilities on the other hand are completely non-supportive. Sadist requires that he get last hits and kills for regen. Heartstopper aura works best by scaling into the late game where enemies have high HP and he needs to stay alive to do that damage over time. His ultimate is his sole disable and since any kills in Reaper's scythe are given to Necro you would just be taking kills that your carry would be better off getting.
A level 1-3 Necro can't really do anything. He can't gank effectively and his abilities for keeping a lane partner alive aren't as good as Abaddon, Omni or Dazzle.