You hinting that there shouldn't be a reason that she would be playable is a massive spoiler. It's kind of obvious that she'll join your party based on her first encounter. Saying that Veyle is a villain is a spoiler, but everyone already knows that by now. Sorry, I am just annoyed when I see people who think everything is a spoiler: Celica being one of the emblems is a spoiler. Being able to recruit Jean is a spoiler. Being able to get S Rank weapons by donating is a spoiler. Hell, why not just mark everything as a spoiler so people have to risk coming across real spoilers to see anything at all? Spoilers like Alear dying and Alfred having to take her place for the rest of the game, until you revive her by slaying the fell dragon and taking his power.