There’s a lot of stuff besides the extra outfit slot they need to work on, but it would indeed be nice to have!
Perhaps the slot could be called “tea-time”?
It’s because morningwear is worn during the working day, when the well-to-do are doing serious brain things, while eveningwear is worn after everyone’s clocked off, when they’re going to dinner parties or dances and generally charming it up.
From a symbolic perspective, evening is the transition from light into darkness (obfuscation), whereas morning is the transition from darkness to light (illumination). Watchful, which is often tied to learning and discovery, is connected to the time of day when our sight becomes clearer, while Persuasive, which is often tied to charisma and manipulation, is connected to the time of day when we would band together for security and our vision is darkened.
It still won’t stop me from using Evening as my Watchful slot and Morning for Persuasive, though.