That was just one episode, she has to be in all the episodes with the rest of the CMC to make it OFFICIAL. So Babs Seed doesn't count. CMC is better off with just three and everyone prefers to keep it that way. Besides she doesn't live in Ponyville.
Okay so somebody added that to wikipedia but i still say she isn't so there. Since she only appeared in two episodes. Now that there's not going to be anymore episodes of her appearence and the only 3 CMC are always in Ponyville. Who cares what wiki says.
It is not important how many episodes she appeared. She don't have Cutie Mark, she will help other blank-flanks to get they CM - so she is member of CMC. And it is not important where she lives. She was in Ponyville, she is crusader and she will be always in CMC. You said that she must be in all episodes with rest of CMC to be crusader. It's so stupid! Eg. Rainbow Dash is not in all episodes with rest of her friends. So she is not in mane6? Of course she is. Babs Seed is in CMC. OFFICIALLY