Is it a whole gameplay mode or just an "extra episode"?
Honestly, it doesn't seem like a real deal-breaker, since you still can't use Iori, Ami, Azusa, or Ritsuko as members of your own individualized unit...you'll just be playing FESes with RK instead of your unit, right? and if the storyline is the same as the main game (as in, being un-skippable, and the same things happen every playthrough more or less)...
Well, it'll be nice to have a little extra Ami, Ricchan, Iorin, and Azusa, nonetheless~