That's not completely true. I've leveled a shaman to 55 recently, and with a Barbarian Spiritist's Hammer it certainly seemed better to tank sometimes ... or at least it did before the nerf (I haven't been using mine much since the nerf so I can't say for sure now). Similarly with a JBB it can also make sense to tank, because it breaks root frequently.
With just a poison wind censor and no BSH or JBB though, I'm not so sure; OP really needs to do the math if they want to know for sure whether tanking is a good idea. It might seem like rooting always wins, since you get more mana and take less damage, but you also have to consider:
* the mana cost of root itself
* the time it takes to root
* the damage you'll still take (while rooting, when root breaks, etc.)
* the melee damage you won't deal
So it's not necessarily clear-cut.
P.S. To get back to the OP's original question:
Per-Level Hunting Guide