It is true that there are many other healing allies that are cheaper and can heal more than just heroes, Daughter of the Nimrodel is actually really good when combined with Ent heroes. As of this writing there are only two so far (Treebeard and Quickbeam), and both have abilities that are triggered by damaging them, and both abilities are really powerful. You might think sticking self preservation on those Ents would be better since it's harder for the Encounter deck to get rid of attachments than allies (usually), and for the same cost. And you would be right, except for the fact that Daughter of the Nimrodel has something in exchange for this vulnerability: the ability to heal fellow player heroes, and the ability to heal either Treebeard or Quickbeam depending on your current needs--i.e. a little more versatility than Self Preservation. Using these two in combination (Daughter and Self-pres) is really nice.