It's not so bad. At best, you could sack a spare Hector (if you have one, hence "at best") and give her Distant Counter and the Distant Defence seal, and now she can deal with mages better than any armour other than Sheena; at worst, you can give the Slaying Axe+ or Armour March to someone else, such as Sheena.
Not sure what your definition of "heavy investment" is, but she comes with her best weapon by default, which is usually the most expensive part of any build. All she needs is an A and B skill, and a special, and all her best options for those slots (Fury, Desperation, and Draconic Aura, respectively) are available on common as muck 3-4 star units. Unless you're referring to merging her, but that only really matters for high-level Arena play. Or getting someone to buff her, but Eirika is in the 4 star pool so she's not that hard to come by these days.