Suppose you level shield first and max it at level 9, (which is often when midgame begins to hit) Karma's AoE mantra+shield shields for 300+ even without any AP, which is basically a stronger Locket + Shurelia.
And even after you use up your mantra charge, you can still help peek for your AD carry with Q + W combo to root people trying to dive, and/or use shield to speed them up to help kite. --- http://arwentinuviel.deviantart.com/
THe problem is, if you use it to start a fight, you've wasted the damage part of it. If you use it during a fight you've mostly wasted the MS component of it. Her skills set her up for a lose lose. --- What do you do when faced with two choices? Simply toss a coin. It works because in that brief time that the coin is in the air you know what you're hoping for.