Heierrat Mage is an amazing archer unit for level 3. Compare Nightwalker / Riza to her. In a Merc Nation, she'll almost always have more RNG that Riza. Other than that, their stats are pretty much the same except that Mage is 10hp lower, but also an entire level under. Riza is a decent unit to play because of the big damage, but his nighttime skill is conditional and rarely goes off unless extra resources are devoted. Once ranked-up, Heierrat Mage may simply serve as a big damage archer - the action skill may even be considered an added boost. It is a devastating skill against level 2 rushes, often very useful because she's generally safe behind other tanks, and when they are taken down through use of Cemetery Rats, she may in turn wipe the opponent field.
Just because Heierrat Mage doesn't hold up the name of Gowen beatdown to extremes doesn't mean it's not playable.