How important are fast aerials/double jump aerials and what is the best way to practice them?

Diamond 3 here, I have a buddy in diamond 1 and we pretty much exclusively play doubles with each other. We've gotten a little stuck in this diamond range and unable to move up so we've been discussing our skills and strategy a bit. One thing I've pointed out to him was I noticed he still does not take fast aerials when appropriate and that it would be extremely beneficial to both of us as a team if he were able to.

However I'm met with a little resistance that he just "cannot get it down and the risk of back flipping isn't worth it" in these game situations. I'm still trying to convince him to just drill it into muscle memory but I don't know the best way I'd try to approach it as someone who doesn't do it out of habit, as I've been doing it for awhile now.

Also, I've been telling him it's important for our games but I'm struggling to convey exactly HOW and WHY it is important to learn, besides obvious situations such as saving high goal shots. And quite frankly, we managed to get to diamond together without him fast aerialing so I don't know how much of an absolute necessity it is as a skill, and I don't want to be too pushy about it also. So what do you guys think?