First of all, I have to say this: I didn't like the episode, for a lot of various reasons that I am going to write down here (in a, I hope comprensible way). The first thing I have to talk about is Babs, why does she get her Cutie Mark? There was no need to do that, the writers could have delivered the same message without doing something this big, a character discovering his purpose is a big deal, and we should have got an episode dedicated to her, to show how she got it and how things are gonna develop for her, (even if for my own personal "head-canon" the four of them should have got their Cutie Mark together, their quest is all about that, it's not a quest for fun or anything else, it's a quest of self-discovery, and if they started together they should have finished it together.)
The second point I have to bring up is "characterization", why are the characters so poorly and badly written?? Ok, it's a dream, Luna controls it plus there is AB mind going wild, but the CMC sending away one of the crusaders is outright stupid, they would never do that, and if they throw out one of the members because he/she has got their Mark it means that their friendship revolved around the Marks and not them as "persons", it was a mere "objective" friendship, they didn't understand friendship in the first place, and this is another thing that is most likely to not happen, or at least it will not if the script is done by someone who knows what he/she is doing, picking up from my head-canon once again, I always thought that once one of them/all of them get their CM they will just help others find theirs, that their friendship would be so strong at that point that the CutieMarkCrusaders would become a much bigger thing all thanks to them, that would be a great ending to a great story of growth.
My last point comes from the end of the episode, "Even if she has her CM we can still be friends", yes, of course you can, a butt tattoo doesn't change the person, and right after that AB clearly says that Babs isn't a crusader anymore, why?! Why can't she help her friends and others to find their Cutie Mark? This is simply poorly thought writing, just as giving Babs Seed a Cutie Mark in the first place is, this episode could have been so much better, it really lacked in the writing department, and AJ singing or the smooth animation can not fix that, this episode is simply an error, a joke. I highly hope that the letter was a fake, if it isn't things would change irreparably, and I'm sure that no one wants that, not in this way at least.