If you want a very expensive army that will make others mad with ya.
But seriously don't know. Haven't played against the current Wood Elve Army Book since World of Hobbies and Games. Even then at most they would field one unit of Treekin. If I were to speculate a couple of units large enough to cancel ranks and take a couple of hits could get ugly, kind of like how many Ogre players probably imagined how their army would fight. These guys are as special slot, right?
Dryads to manuever, maybe force certain manuevers out of the enemy, these guys could try to lock an enemy unit with a charge. Next turn the Treekin and or Treeman slam the enemy, preferably in a side charge to even out combat resolution.
One really big disadvantage of any wood only list is the lack of rank bonuses. Warhammer Fantasy Battles is all about the combat resolution, you can lose the killing game and still win the fight with ranks, banners, and flank charges. My rank and file undead can't kill jack, except the skeletal horses who sometimes seem to kick more butt than the Wights.