Public reading filters can have some use! Like, for instance, I have a Dreamwidth reading filter that I'd be perfectly happy to share with others. They are even more useful here than on LJ, because paid accounts here have the ability to make super special reading filters that free users can't, and if they could make those filters public, the free users would be able to go read that paid user's compilation. (Of course, that could be an argument to make filters NOT usable publicly, ha.)

I think the minor loss of privacy you mention is going to be a much bigger deal to some others than it is to you, especially towards lurker personas.

It wouldn't be much work for a person to figure out whether or not they were in your default reading filter--all they'd have to do would be to make a public post and then check your reading page. That's a pretty low bar of discovery for something some people might not want to share.