For me I felt like the whole adoption showed up how sloppy they were already being. Because she's a great character and had a great history and relationships, but it really wasn't a history of being Bruce's daughter or a sister to the others the way that Jason, Dick and Tim always had elements of that with each other. It felt more like Beecham had this idea that at the end of his arc he wanted Cass to have proved herself and wanted some symbol of that and chose Bruce saying "I'm starting adoption proceedings!" Unlike with the boys where he really was saying "I consider you a son" or "I will be a father to you." It's comparatively weak.
(In fact, didn't he even say "I'm starting adoption proceedings" where with the boys he sort of respectfully *offered* because he almost felt like it was presumptuous? Except maybe with Jason where he did it a lot more quickly, but again it was a way of saying he really was going to be there for Jason and not let him down like everyone else did. With Cass I don't remember he asking her.)
It's funny, but writing that makes me think of that retcon of Babs where her reward is to say Dick's vow and it's the same type thing. With Dick it's character-appropriate and we know what it means and it's just giving words to a relationship we can see and is ongoing. With Babs it's a reward that says "Okay, you have won my approval." And that's the end. The vow that meant everything to the little boy meant a lot less to the young woman.
So since Cass had really never been written as a sister or a daughter (that I could see) to these men her adoption's doomed to just be this empty gesture. Which is a shame because they could have concentrated on what she really was. Instead it feels like the end of the arc tells us "Well, she's NOT a member of this nuclear family..."