........ Why?
Because we should be punished for wanting to work on more than 1 Magian weapon? I'm sorry i see absolutely no reason why Magian weapons, Weapons designed to reward us for continued effort, Should be limited to Main hand uses only. Its really quite a silly idea.
The worst thing in the world SE Did was add that terrible "Main Hand:" things to those Fey weapons. It was basically a giant middle-finger to 1-handed DD. Doing it to Magian weapons would be no different. There is no reason outside of disgusting jealousy (Waah they can dual wield and i can't, you should nerf it). Things should never be nerfed because someone is to pathetic they can't let other people be happy with something and toss around lame excuses like "balance". (Not saying this is your view, just the only one i can think of. So im not saying this describes you)
on the topic of 2-handed weapons, their bonuses are actually slightly higher. the Shareeravadi+2 for example has Pet PDT-12%. Its not much i know... But its a little improvement. Personally I think 2handed weapons should have gotten at least 1.5x the Stats, Since its impossible to Dual wield.
That or Magian trials for Grips to make them as potent as dual wielding. Who wouldn't get a Grip with VIT+2 Pet:PDT-8%? I know i would go for it. Sadly adding Magian Grips will probably not happen.
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On the actual OP's topic, I have to say i suspect SE actually thought those weapons would be "too powerful" to dual wield. in reality most of them suck terribly due to horribly low damage and horribly high delay, and the fact the Oa1/2/3/4 Spread is something terrible like 40/30/20/10.