1h magic weapon had Acumen as Special Ability in original chronicles, that's why nukers prefered these weapons for pvp (Homu, Valhalla and Miracles), and that's why nukers prefers low grade sets for pvp (Karmian or Avadon), casting speed is the main stat. Two handed staffs never had acumen, so this type of weapon were avoided.
And of course, what else you gonna equip in the offhand if not a shield?
In Classic, you can put the whatever special ability you want on any weapon, you can get 2h staff with casting speed. So the question is if shield is giving to a nuker more survivability in exchange for less damage. Is something each one has to check, go to arena and see for yourself how many attacks you can block with buffs and without buffs. You can check it with the cheapest shield, more expensive shields only give more defense, not chance to block.
I think a shield doesm't make difference for a low Con class, but is something that has to be proven.