1. YOU MONSTER why would you want to do that to best girl?
2. She is kind of useful? for teams with more *cough cough* elemental variation.
3.Sadly enough according to the wiki despite SSR versions of the main characters popping in a couple of events we have no SSR version of diabolos so far, it would be so cool though!
I also hope at some point they do something with Diabolos. I kind of get why they don't want multiple copies to exist so you can limit break the normal way. But feels like they should do something creative. Releasing an SSR version would be nice. Would also approve of them just changing the system a bit so she can go through limit breaking in the same way as Kamihime. She is unique in being a main character in the story. So just have her not function the same way in terms of limit breaks as a normal summon would.
I guess the devs really only saw diabolos as story fodder (for the purpose of contrasting with the other heroine who´s name i forget) and "adequate SR for new players" and didnt go beyond it.