Should exist in the game, even if they aren’t all that great at being either. That is all. 
Not possible currently due to how item types are determined, unlikely crate will change that so…
The better route is just remove skills/proc limitation of having to require an offhand to begin with.
I mean they have removed plenty of requirements already for some skills
Not gonna lie… having AAR being channeled through your shield would look pretty awesome. 
Kinda like a skill from an old mmo I used to play back then, where there was this skill that made you fire a huge energy beam from your shield.
Except for devastation, they should keep the limit
Personally I dont care either way was just letting OP know the better suggestion
Tho I do wish the Devastation proc on Outcasts MI Hats worked without needing an offhand
what is the best: soldier+arcanist pistol & shield or demo+arcanist pistol & offhand ?
Defensively Pistol & Shield
Offensively Pistol & Focus as you get Devastation
Didn’t know this. Oh well! Removing the limitation is the best option, then.