The accepted wisdom:
1. Taunt doesn't work very well until at least the 20's. (Either because of
lack of skill or lack of mob intelligence).
2. Snare and root both work as taunts.
3a. Snare is usually preferred to root, because the mob will still run when
low on health.
3b. Root is usually used to park a mob out of the way, or when you want to
run away.

Root has a specific use which I have found very useful in the teens and I
suspect for a little longer yet. In my experience, a rooted mob will attack
the nearest person. Not the person who has done it the most damage, or the
person who has taunted it a lot, but the very nearest person, even if the
people who have made it angry are still within melee range.

I'd be surprised if this is a special effect of root, it probably just drops
out of the mob anger calculation, part of which is based on distance. But it
seems to work. Here's a specific instance:

The other day I was in Crushbone and a couple of people were attacking a
centurion. They were levels 8-9. When he got to to about 1/2 health I rooted
him, and stepped in closer than they were. I didn't attack the centurion,
but he turned to me and started hitting (OK missing) me. This could be
attributed to the taunt value of root so far. But when one of the other
people moved closer, he switched back to them. Then I moved closer still,
practically on top of him, and he switched back to me and stayed on me for
the remainder, even though I was never attacking him.

I've found this a good technique for damage-spreading. But don't take my
word for it - try it out. My experiments haven't been very scientific up to
now.

Elbeno, 19 times blessed by Karana