I think there is a point to be considered here. Opponents, unless they are familiar with the spell, won't necessarily know that cover or distance will end it. If you have a character in the party whose player who hasn't read the spell description and is targeted with it by an NPC, is that player automatically going to have his melee character just move 30' away so it turns off, and then charge back into the fight? I'm guessing a lot of melee warriors are just going to try to keep getting into melee with that caster, and if they aren't able to get past the goons and meat shields, perhaps pull out ranged attacks and go after them. A lot of opponents won't know how it works either, which means it can be quite effective if you can keep them from closing with you.