Not so much not worth hanging on to as "Frumple doesn't bother with it"

That said, everything except particularly lucky damage resist rolls obsolete pretty easily... there's some level of influence based on material level (so, ferex, a copper gladiator ring will never [caveat: so far as I know] get 10 on its stats, while a voratun glad ring can.), so it's not uncommon to replace older stuff fairly steadily. There's pretty comfortable tier breaks, and as a rule anything at least two tiers above an item is going to be noticeably better -- it's next to impossible for an iron weapon to beat out a d-steel, steel to be out stralite, d-steel, voratun. Stralite and lower quality voratun stuff is more equitable, but you'll probably (eventually) replace everything with fifth tier stuff (barring certain artifacts). Similarly the best iron can beat the worst steel, best steel worst d-steel, etc.
As a general rule, just try and keep whatever the "best" for a particular slot or stat is, if you're going to go through the effort. It
can make many encounters considerably easier if you kit up for them. I'm just too lazy to go through the effort of swapping crap out

Anyway, as you play more and see more crap (or code dive the objects folder and see what the maximum stats are

), you get a better feel for what's particularly rare or unusually powerful or useful.