So the more I think about it, the more I realize that Flex is literally just the new Monkey Grip. In case you're not familiar with it, Monkey Grip was a terrible Feat in 3.5 that allowed you to wield a bigger weapon than usual in one hand. It was one of those feats that you can prove with math is worse than not having it at all, but I've run into so many players who adore it (and several DM's who bemoaned how "overpowered" it was). Some people just want the fantasy of bigger damage with one handed weapons. and don't care what hoops you have to jump through to get it.
Should Flex be better? This is a tough question; on the one hand, I want to say yes, but on the other hand...if we want players to use masteries to make combat more tactical and interesting, I'm not sure how making Flex do more damage will get us there.
"See, you can use this d8 weapon to knock people prone and give your melee allies advantage."
"Or...you know, I could just do 1d12 damage and kill things faster! Best status effect is dead, amirite?"
"SIGH."