I think it's fine, to have relays with different levels of access.

Maybe we are writing things that we don't want everyone to read, or storing things like drafts or events addressed to a particular community. Or maybe we don't want to open ourselves up to constant ridicule and threats, or just want to talk amongst ourselves. Or we want to have more control over the ability to edit or replace events.

AUTH on reading doesn't stop information from spreading (there is no technology that does this), but it slows it down, considerably. Feels more like writing on Telegram. 

 That said, I tend to cross-publish most events to public relays. If I want a wider audience to read something, I put it in a place where they can read it and respond.

@cloud fodder is working on a way for us to be able to write everything to theforest and then categorize it public/private by kind or tag or whatnot, to determine if it will be auto-broadcasted to "public" relays. 

 "Public" in the sense that they don't require AUTH to read, but they're usually write-limited.