It's fine for one or two cards, but for all the rebate stuff and membership benefits, I would have to carry around a veritable mountain of cards. They already have all my relevant details for the memberships, and I don't share anything further than that via my phone.

Posting anything online is a sacrifice of privacy. Using HN means your IP, your username, a bunch of connections and other data are logged in various places. We're all willing to sacrifice privacy for little conveniences, some people prioritize differently.