Well the training event is at least a chance to level the new girl and someone else for pretty cheap whilst you collect the potions to buy Kyoko mats, granted you might not have anyone you actually WANT to level...
The episode isn't out until the 23rd, although I'm enjoying these 5* kyouko supports. FWIW I seem to get 5 potions from the 20ap enhancement stage, haven't tried others yet. So that's 40ap for a ribbon, they're 16 in the store. It's 64~70AP average for ribbons off the story mission, and at a 4ap per potion average it's... 64 AP per ribbon lmao.
So yeah, unless there's literally no one you want almost-free huge exp on, you should totally use the event. (and PSA:You can buy the girl for 16 potions, which is only 64ap, so don't be like those people on reddit who burned 20+ pulls trying for a 2* char)
I don't think it's getting double rank EXP though which is LAME. I understand Diary didn't get it b/c it didn't cost AP so it'd be busted as hell, but for this event it's like, the event is great but I have to miss the OTHER event to do it...
edit:According to the wiki Mitsuki doesn't even use the ribbons ever, so they're purely there as a mercy to kyouko users along with those fire orbs LMAO