This is just wrong. My shadow is geared to maintain aggro against DG sentinels who go balls out from the get go of a fight, while still maintain the 30/65/60 mitigation stats that is expected of me. I haven't guarded a dps in quite some time, 50-50 laziness/lack of need. While not all tank classes are created equal where aggro generation is involved, but a blanket claim that tank aggro cannot keep up with DPS aggro is fallacy.
PT's and Assas/Vanguards and shadows may not have any problems, but on my jugg tank aggro is a whole other thing.
AOE tanking abilities: 2, one on a 15s cd and the othe on 3 mobs, if the mob your hitting with it has 5 stacks of sundering armor.
Aggro generation: very low, because we don't do as much dmg as the other two tanks.
So I have to guard a dps (especially PT's as they don't have an aggro reduce)