I kinda liked Ashley. Plus I didn't protect her half a game and spend my precious yellow herbs on her, just to never hear from her again. Besides they are expanding on Leon's bond with the president. He must have seen Ashley regularly if they were friends. And the death of her father seems like something that makes her character relevant. If Hunnigan reappears, Ashley will surely do too.
Finally, imagine you had to shoot your best friend. I'm pretty sure you know he wishes for you to make sure his daughter is save. It wouldn't make any sense if Leon would just walk away and not make sure Ashley is not being eaten by zombies.