He is. Omniknight is actually just an all-round supportive character. He heals (His heal hurts enemies as well, if they are close to the target you heal), he grants allies magic immunity, he snares enemies around him with an aura and his ultimate makes every allied unit and hero within its cast radius virtually immune (99% Reduction) to physical damage and grants them crazy health regeneration for its duration.
He does suffer from an arguably weak laning presence, early game mana issues and rather long cast animations.
Having tried out a champion in LoL yesterday with horribly long cast animations and early game mana issues, I think I can cope with that.
Just need to find where the damn secret shop is when going from bottom to top.