Huh, I’ve never really thought about them. If they are a rusted mess, they get pulled, if they’re in good shape they stay. Good place for a speaker!
i use them sometimes but they’re small, water gets warm, and they get covered in road grit. Camel back all the way for me. Mine is insulated so I throw in some ice, water, just a touch of lemon. They get emptied and stored in the freezer so they don’t get gross between uses. Works a treat.