I agree with the idea of a bestiary, it's a fun thing to try and fill up. However, rather than something you have to waste time casting a spell on (wasting ability slots for a basically cosmetic feature), why not have it fill up the more you engage an enemy, like FFXII's bestiary. After you kill enough individuals of an enemy type, more information about what it's weak to, level range, likely drops, etc show up. Writing the lore for each enemy like FFXII did would probably take up valuable time from the writers, but I think this information would give a great incentive to go kill, especially if you attach a few achievements to it.

However, I do think very basic UI information like aggro, enemy level, and possibly linking behavior should be shown at a glance, although the current implementation is a bit odd and should probably be adjusted. Also, if they do add achievements for the bestiary, it should be for a concrete amount of enemies, not a percentage of the population, i.e. 10-50-100 etc types of enemy bestiary entries filled, not 25-50-100% of enemies in x area, simply because it's very tedious to hunt down every last type of mob.