Some of the really old stuff use corrosive mercuric primers which will eat up the bore if you don't clean the rifle very soon after firing.
BUT I just sold a full box of 6mm Lee-Navy for $600 so some of the old stuff can be quite profitable.
True, and even some recently produced ammo has value to collectors. Back in the '90s, I bought a bunch of PMC .38 Special ammo that was on sale ($6 a box), for 'plinking' with a Marlin .357 carbine. If I remember right, it was an unusual design with a 66 grain tubular copper "ring airfoil" bullet; shot so flat in the Marlin that I ran out of adjustment on the rear sight. It wasn't produced for very long, and a few years later, I sold the few boxes I had left to a guy who worked at Aberdeen Proving Ground, for $50 a box...