User blog comment:Dj0z/AP on supports - quick analysis/@comment-4922088-20120508001324

I think one of the things missed by just listing AP is the fact they have such varying utility.

Taric may have the highest total ratio, but he has about the lowest overall utility if he builds it because he has to get point-blank next to an enemy to deal all of his damage.

Janna and Soraka are on the strong side, lots of utility and can be very scary when built with AP, but their cooldowns make up for it.

The reason people put Lulu in mid (and Karma but I'm using lulu since you mentioned her) is because she can harass without taking any damage at all. Also, she makes for a very easy lane to gank.