I agree with you guys. General pubs think he should be a main carry (even the majority of reddit thinks so, lol). That only works in pubs and he lacks so many things real carries have (mobility, consistent slow/disable, reaching nuke, etc).
I've had my share of going 13-2-10 with him by going mid + bloodstone/hex/shiva/etc. His carrying ability comes from that fact that his heal/nuke is so expensive & restrictive, but if you have lots of mana, you can spam it and be very very oppressive. Also his aura and ultimate scales forever.
But he simply doesn't cut it for real serious games.
He's awkward because as {bad} pointed it out, there are far superior 4-5 position supports with reliable stuns & positioning skills. Lina/Sand King/Venge/SD/Lesh/Jak/Rubick are all VERY scary when they come out of smoke to stun-lock you to death. And they need 0 items. Necro isn't so much.
I think he should be played as 3 in farm priority, not quite 2. He needs SOME farm then he's real nice to semi-spam his heal/nuke (drums, arcane, force). He doesn't need more as his aura & ultimate don't need items.
He's comparable to... Brewmaster, Silencer, utility Sven/Naga, or secondary core Jugg. Give him SOME gold to make him shine, and that's it.
All that being said, he's almost totally ignored in current pro scene because of the bad reasons I've listed.