Good discussion here. I have a L27 froggie Zerker, and he's very fun to play. My main is a illusionist, so it's a big change of pace.
In my experience, the descision to go 2-H, 1H and shield or Dual weild is infulenced by situation, group, and Loot. Loot is a huge one. If you come up on a 2H crushing weapon that is wayyy better then any of the other weapons you have, you'll likely end up using it. i remember in EQ1, when my Frog pally got his Ardent protector, a 2H greatsword that kicked major ***. It gave me such good stats, i was better off with it then going 1H and shield. In EQ2, the diffenence in gear isn't quite as drastic, but if you're not rich, access to weapons is a major factor. I didn't start using a shield till just recently, because i didn't have a good shield. After finishing Bloodsaber quests, i have a kite shield with 330 shield factor, and nice stats. Even though ideally i want to DW, My 2H imbued sword is much better then my DW weapons, and shield is great for tanking, so I alternate.
Group is big too. if I'm in a full group with 2 healers (or 3, like a group I was in recently), there's no point going with shield. 2H all the way, carve a path of destruction. the group needs more DPS(and keep in mind that zerkers are great at AoE DPS, as well as healing in combat), and the healers can keep me up without breaking a sweat. The extra DPS helps me keep aggro, and yes it does bring the mob down faster. In a group with let's say 1 healer and a few DPS classes, I go with shield and def stance, and let the taunts rip. There, it's best for me to lose a lil DPS so that the healer can heal me easier, since i need to stay alive, and DPS isn't an issue.
Finally, situation. Certain mobs have resists to types of damage, as hopefully everyone knows. I carry DW slash, pierce and crush, 2H slash, 2H crush, and 1h slash 1H pierce. If a certain mob is invlunerable to crush, I can switch to 2h slash. Or just start out DW 2 different dmg types, see which hits. So sometimes, my best option may be to go with another weapon, espically if i only have 1 1H dmg type.
Prety much, i think each tanking class is a little different. Paladins and Guardians excel with shields, Monk/bruiser dual weild or 2H and avoid, SKs love shields and 2H, and berzerkers can do all 3 well. Even if a zerker is taking more dmg from not using a shield, he's still probably regening health at a steady rate, and keeping agro on the mobs. When it comes down to it, it's really up to the tank. Good tanks will keep aggro and keep you alive, no matter what kinds of weapons they use.
But every great tank knows, when you're going up against the most difficult of enemies, always bring a shield.