Should I keep good items I cannot/will not use?
I keep running across enchanted items I have no use for. Such as:
- Armor that does not fit my race
- Weapons I have no intention of ever using
- Scrolls with negative effects
- Duplicates or items that I have better versions of
What should I do with these? Keep them in my stash? Leave them? Can they ever become useful later on?
The items you list can serve a specific purpose; despite the fact that Crawl doesn't allow you to see items, these items can come in handy in certain situations.
Items you do not intend to use can serve as "protection" against effects (usually creature-based) that randomly affect one of your items. For example, keeping a ring of regeneration when you are a troll means that when you kill a mummy, the curse is less likely to hit an item you intend to use.
Duplicates or weaker items can be useful in two ways: as "protection" against the effects of certain attacks and as replacements for damaged items. If you have a +3/+3 long sword that gets damaged three times by acid, that +2/+2 long sword might look a little more useful to you. (Alternatively, you can carry the weaker weapons and use them in situations where they might get damaged.) Note that as of 0.15, weapons and armor are no longer permanently modified by corrosion, so carrying multiples for use against jellies, etc. is no longer necessary.
However, you don't need to carry them if you only want them as replacements. It'll work just as well to leave them and come back for them if you need them.
There are also occasions when items that are useless to you suddenly acquire a purpose. For example, if you decide to follow Elyvilon, the weapons you had that you didn't care to use can boost your piety (if you go back and pray over them). Again, though, it's not necessary to carry them; you can simply return to them and pray.
Keep (almost) Everything!
But do not carry everything with you. Keeping a stash of all of your items is an important strategy in Crawl, as it allows you to explore without running out of inventory space and ensures that your items stay safe from cold and fire attacks. If you are unfamiliar with keeping a stash, the standard first two places are the Ecumenical Temple and the second level of The Lair, as those are both locations where your items will not get picked up by a wandering monster.
Regarding the (almost) comment, a number of items are fairly useless and can be ignored, such as scrolls with negative effects (their only use is to train spellcasting to Level 1, which maybe irrelevant depending on the character you are playing) and armour that does not fit you. However, I like to keep everything together, so that when I search for "scroll", I know that anything not in my stash is something I missed picking up while exploring.