Thorn Wall
I was initially having Thorn Wall Maxed up and using it since I got it. I found it more useful in PvE then PvP but the cool down was insanely long and it requires a fair bit of mana. It does very minimal damage as you have mentioned - about 15 for 3 secs. I found this to be a good compliment after blast shot in PvE, as mobs target after you use Blast Shot.

Thorn Root
Though I disliked thorn root initially, I have come to compare and appreciate it more then its AoE brethren. The trade off - single target, less then half of the mana, faster cooldown, 25% dodge for AoE and minimal damage.

Why don't use both:
Personally it is a bit redundant and though by just putting 1 skill point in Thorn Wall, I can take turn between the skills and keep my opponents rooted, I figured it would be a waste as most melee targets have ranged attacks as well and I don't like crowding up my screen with skills. I figured that 1 point would be better invested elsewhere.