I don't agree that a tanker shouldn't attack. If that was the case a tanker wouldn't have the secondary powerset.
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Actually doing damage is very much a secondary thought. Tankers damage attacks tend to divert away protecting the others, so it's often frowned upon.
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While is is true that they don't do the greatest amount of damage, every hit that a tanker lands on a target adds to the agro control by taunting that same target and those around it. This is facilitated by the inherent power 'Gauntlet'. It's not as strong a taunt as the dedicated 'Taunt' power, but every little helps. I've never encountered any point in which a tank attacking was unwanted or frowned upon. I can say this from both tanker's point of view and that of other ATs.
It's all a matter of player preference at the end of the day but to write-of a tanker as a damage dealer would be hasty to say the least.