Unlike commenting and posting, which offers the who, what, where, and when parts of the message passing process, voting on Lemmy (now, for non-admins) is inherently an unequal process. Imagine if someone could send you an email whenever they wanted, but you were prevented from knowing who or even from what instance it is from, or when it was sent, do you think that could open up a potential for some variety of abuse? Or texting, phone calls, showing up at your door, etc.
Knowing the identity of the voter is an important part of properly receiving the “message”. It also increases freedom of choice, b/c otherwise the only way to prevent such messages (if, let’s take it as a given that some people find them annoying) would be to turn off voting entirely, either by going to one of the instances that does that, or just ignoring all (down-)votes yourself.
If we want the Fediverse to grow, and in particular to include less emotionally stunted humans that actually care when someone says something about them, good or bad, this will be a necessity. (Also, I was speaking tongue-in-cheek there, but genuinely social standards do vary across this wide world, and it really would increase content if there were not only more but different types of people, especially those most likely to generate quality content.)
And as other non-Lemmy methods of access to the Fediverse provide that feature - k/mbin, piefed, sublinks - Lemmy will fall increasingly behind if it were to ignore this very basic feature.
Making the votes public also increases honesty, since they are already public now. And if you don’t want to know who down-(up?-)votes you then… don’t look? But for those who want to know, it will be a great feature to have.