Originally Posted by General Sassycat
Decent heal, decent shield, decent slow, decent damage. The only thing she seems to lack is the buffs that other carries have. Plus if i wanted a mage i'd rather any other mage than karma. I'd rather AP yi mid.
That's the exact problem she has.
Her spirit bond is okay for a speed buff, but sona's is better overall (and is much less situational when it comes to buffing her team).
Her heal is okay, but drops off incredibly fast without any AP to back it up unless the targets are critically injured with high max hp. As a nuke it's too short range to really be practical early game, and late game it'll deal insignificant damage.
Her shield is okay early game, but it has no utility unless you spend a mantra charge for the aoe nuke - except you have no AP so it does no damage. Using it for damage in the lane will also push the lane hard. Late game it's not really that good as a shield either, with no AP.
With AP, her Q and E scale pretty well, so she can remain strong and still offer support options (and her shield will block a silly amount of damage if you're nicely farmed). Without it, she just can't perform on par with the standard support options. Can she do it? Sure. Should you do it? Not unless you're bored, or you have a melee-centric team (to make spirit bond do real work rather than essentially being a 2 target speed buff)
Buffs, sustain and CC are what define the core supports. Karma has passable sustain (once she has enough CDR to get mantra charges quickly), but she has no good buffs and she has no CC either. All the common supports are strong on at least two of those points. Some go particularly top heavy in one point (like Janna). All supports are USEFUL with no items. Karma...really just isn't that strong without farm.
Incidentally, AP Yi has nothing in common with Karma. As a mage, Karma offers a good heal and a strong shield. Yi just has a single nuke and the ability to bait the enemy team with meditate. He has no utility. If you REALLY MUST play Karma as a support, then go ahead. But don't expect people to be thrilled at the prospect.