I regularly group with a monk. He pulls, tanks, and does dps well. In terms of healing, he doesn't need much. Mend helps a lot, as does his iksar regen. He holds aggro well enough, and if not, root isn't much mana.
Sometimes we'll get a tank to join us. There is a distinct difference. While the tank does require more healing, his aggro allows me to med more. I usually have plenty of mana either way.
Now, when the monk pulls a wizard, that's another story. He has pretty poor resists. Usually the tank can stun and their gear has more resists. That's the kicker. If you're fighting casters, the monk usually isn't as good an option.
This is in smaller groups (2-4). In a full group I don't think that would matter as much. The monk holds aggro over the wizards when we invite them. Heck, the rogue can keep aggro over the wizard.
If you're not fighting raid mobs, the difference is negligible. It's all about knowing your group and their potential. Paying attention helps exponentially.
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