It's more complex than you think. The only off hand items we get right now are, for all intents and purposes, stat sticks (the block value/rate are simply stat increases to hidden values). Weapons actually have unique values that other stats modify (i.e. weapon damage; everything else damage based is purely a multiplier to weapon damage). On top of this, all weapons have normalized damage values: a sword and an axe of the same i-level deal the exact same amount of damage; auto-attack damage might be different (to account for differences in attack speed), but the damage dealt per potency is exactly the same. Adding in an offhand weapon that also damage stats would create some balancing as well as mechanical problems: if the weapon damages are added together, even if one is added at less than full value, you end up being able to do substantially more damage per potency. It also creates animation problems and a whole slew of other issues.
The devs have basically said that, if they do add dual wielding classes, they are, in all likelihood, going to have both weapons as part of the same slot (I believe the devs referenced this when discussing NIN and THF). As such, you would have a brace of weapons in your main hand instead of weapon in your main hand and a separate weapon in your offhand. Even if you clone the main hand for animation purposes, you still have issues with the stats on weapons intended to be used with an OH weapon having only 2/3rds of the itemization as "full" weapons.
I don't entirely agree with this, mainly because I'm not sure that I see the necessary disconnect between a shield and DPSing. Shields don't thematically imply tanking any more than a 2h weapon thematically implies DPS. Shields are a fundamental part of Gladiators: you get several *offensive* attacks that use your shield so it's not like it's just sitting on your arm blocking stuff.Would make more thematic sense than a DPS with a shield or somesuch.
If a DPS class is going to be drawn out of the Gladiator, I fully expect it to keep using a shield. Getting rid of it would require too much work (redoing animations, renaming and completely reworking abilities; balance and mechanical changes to account for gear changes/discrepancies) and there isn't even a particularly compelling thematic or mechanical reason to do so while there are certainly a number of mechanical hurdles that would need to be taken care of it it *were* done.