probably the latter, although most of the time you wouldn't feel it since most mobs just die from normal attacks from your other units if they're weak and bosses seem to be immune to instant-kill (or at least I can hardly tell if clissa actually dies to levi's class + skill effect on dmm account). The primary use of curse users (IMO) are likely the damage reduction (helpful against units hitting more than 1k, and mithril golems and their 5k ass smacking) as well as instant killing mithril golems who also happen to have more than 40k hp.