Tree form were not used in TBC Pvp it was thing on WoTLK, Im pretty sure i remember this right.
“Shapeshift into the Tree of Life. While in this form you increase healing received by 25% of your total Spirit for all party members within 45 yards, your movement speed is reduced by 20%, and you can only cast Swiftmend, Innervate, Nature’s Swiftness, Rebirth, Barkskin, poison removing and healing over time spells, but the mana cost of these spells is reduced by 20%.
The act of shapeshifting frees the caster of Polymorph and Movement Impairing effects.”
Yea, No armor bonus there + moment speed reducion was too much for PvP arena, but yea, as people are more skilled we totally might see Trees in TBC Arena