I've seen a post where Treants are used as tanks during sieges. I've found they make a good tank (distraction) when going up against the AI. I usually put them in front and move them forward until the AI sees them and rushes forward to attack. Then I move in my hero and other melee fighters to engage their preoccupied troops while Calvary rushes to the rear to take out their archers. If there are Ogres (Orcs), or Calvary, I'll send the Sentry units against them as that's their job. The Rangers and Female Wardens sit back raining arrows on the front line which helps them drop faster.
When the hero is fighting the enemy near the treant I use his mass heal to help keep the treant going, and if it gets too hurt I'll send it lumbering to the rear and heal it up after the fight. *EDIT* The Mass Healing seems to be nerfed (or bugged), it only heals for about 80 health points per use. So healing a 22K treant in the field isn't going to happen. *END EDIT*
So far this has been working well against the AI but I doubt this will work against PCs as they would probably just ignore the treant and focus on your hero or other troops.
My troops aren't extremely high level yet so this may very well be a nice starting tactic. I may have to update it when I encounter enemies with FIRE!