One upside to root armour is the mats aren't useful for anything else.

Only you know if you want the bow skill buff, it's not really necessary but if you rely heavily on bow you might want it.

Personally I wouldn't consider root armour just for movement speed, but I might stick with troll instead of moving to iron. The best armour for movement speed is fenris armour, made from fenris hair found in frost caves in the mountains.

Heavy armour is good if you find you're getting hit too much. It's good for entering new biomes where you don't know what the dangers are, it gives you room to make mistakes and a fighting chance of survival. If you can avoid getting hit there's really not much point to it.