I'm not advocating trading out something like Cultivate for Farhaven Elf, as two lands is definitely greater than a land with a 1/1 body in many cases, but I think that ramp is really important in your deck and Farhaven Elf follows with the Druid theme while helping with the ramp.  I see this as much better than a 0/3 Treefolk Harbinger, even if he is only one mana.  The only Treefolk you have to fetch is Woodfall Primus, so if you've already drawn him, you're left with the option of searching for a Forest, but if you don't really need one, you'll do nothing, because you don't want your next draw to be a dead card.  At least, Farhaven Elf puts the Forest on the battlefield, making your next draw slightly more likely to be a spell card in the later game.