It's like 'here's a cool sword. But you can only equip it anywhere outside the combat'. 
I'm pretty disappointed about this too. I mean, if they had to choose, I'm glad they got rid of the beige. But they didn't have to choose. They knew a lot of people also didn't really like the Formal Attire, so why not give us the option to change it there?
And don't bring up that "Oh, they can't, it's too hard on the programming" thing. No. It isn't. The Skyhold outfit is different since it wants to show no matter what. But the Formal Attire is just an armour like any. Last playthrough, I got into my own armour at the ball by accident, just by skipping lines really fast and preventing the Formal Attire from loading. So clearly, it's an arbitrary limitation, not a programming one.
It's also ironic that you get its schematic after the ball. We should have gotten it before and tinted it to our liking. That would have been a huge improvement.

Just look at how much better that is already! Even if the outfit itself could be better, if I could just wear that maroon one my Quizzy has instead of the brick red, I would stop complaining.