Admittedly, as much as I don't think CBs are needed, they can indeed work with or against a side in a time of war PR-wise. Additionally, it also provide entertainment for the peanut gallery.
From another standpoint, casus belli are part of the political side of the game, and you can justify them to your alliance members, allies, other parties as well as enemies. Whether those others will accept your point of view and the legitimacy of your reasons is all-together another thing entirely.
So that is one of the things that is part of the player-brought politics to the nation sim, although it is the onus of other parties to dispute a CB justified against them regardless. Even without a CB, that doesn't prevent an opposing party in a war from disputing why they were attacked, if they can see reasons or not. So, as long as there is salt (and there is an unquantifiable bounty in this throughout the community), CBs, imagined or otherwise, will always be disputed. So Pre-pontification over this point may raise an issue for discussion, but I doubt you'll have an affect on practices as done by the community.
It's good you clarified your point, because I read your post and I still came away with "CBs aren't needed to hit people", which I agree with and in the grand scale of things, it never matters which side is "right" or "wrong", that's arbitrary morality and ultimately subjective, which you've acknowledged in your initial post. Therefore this same subjectivity is applicable to your argument too, since it's also about the validity of CBs and complaining about them, or complaining about complaining about them, as the case may be.