I've tried and tried to come up with a reasoning behind the AM design and I just can't find any. The casting on a player is as likely to be a mistake as it is some intended feature, since the other mechanics of the spells make that feature irrelevant. You can't plant a bunch of am's on a player and have them run at a mob group suicide bomber style since you have to be in range and the enemy already has to be engaged. There's not even an applicable kiting use.

I think the ability to cast on a player was just a developmental oversight, or part of an idea that never made it into the game.

AM would be worth it as designed if it did as much dot as the spell spiked at the end. That's really the only easy way i can see AM salvageable as we know it.