I see what your all saying. I was just trying to figure a way to let the rogue duel wield and add damage to secondary attack without having to multi-class.
Let me say, then, that your rogue would be fine dual wielding without this feat or that fighting style, and I actually think that he's better off not bothering with either.
He's got plenty of other options to use his bonus action on, so if he hits with his first attack (and uses his sneak attack) he's better off using his cunning action. It's when he misses his first attack that the dual wielding becomes useful as a second chance for a sneak attack.
The extra damage from the fighting style just don't seem significant enough to matter. The nice bit about Dual Wielder feat though is getting to use rapiers instead of shortswords, but that hasn't made me select it yet for my dual wielding character. I'm just not concerned about squeezing out a little bit more damage. I'd prefer some unique benefits -
Oh, maybe let the Dual Wielder feat let the character make 2 opportunity attacks in a round, one with each weapon?
Or a second reaction that is specifically an opportunity attack with the offhand weapon. That would be cool.