So back when I was leveling a druid alliance side, I was bitching and moaning about no aoe healing until the mid 70’s (which is bad), but at 15 (when dungeons unlock) is far worse. I had two abilities when I entered my first dungeon: rejuvenation and swiftmend. A hot and a 15 second cooldown heal. With just those two heals, well, you’d better hope that nobody takes too much damage and just rely on the roflstomp nature of low level dungeons to get you through.
I got regrowth at level 18. Now, I could heal a whole low level dungeon with just this one spell. Inefficient and would probably run me out of mana, but at least I’d be able to cast something instead of saying “duhhh 5 more sec guys”.
Compare this to my disc priest who got flash heal as a baseline before even picking a spec and then received penance thereafter. You can easily functionally heal with just those two spells at low levels. (Edit: I should add that even before flash heal, I got the Bubble. Worship the bubble. But it’s not a spammable heal and would run into the same problems as swiftmend.)
The first spell a healer should reasonably get is the simplest: the single-target spammable heal. This is just basic logic. Grrr.