There are a few quest items I have kept just because... i.e The Seer's Ball, the Dwarf Gold Amulet, the Pirate Queen's Diamond.... I kept them for vanity. I have even sent a quest item or two for gifts to my dearest friends.
I USED to be the world's worst at ditching quest items. Quests that were part of a questline broke me of this habit.... Now I check the wiki for each item obtained (the guide doesn't tell you, but the wiki does.... Fix this, BG) Still, especially since WE write the wiki, there are items missed.
And for those of you thinking you'll have to wait days to get a replacement item.... On Christmas Day, I tried to start a quest. Yeah, I had ditched the necessary item (obtained from the previous quest), so I submitted my ticket, thinking I would get my item on the 5th, when the office reopened.... Hoofie himself was here on the 26th, and he sent me my item. They do send them, and they do it post-haste. Nuff said 