While I usually lean more towards the "keep questing interesting by not helping the player too much"-crowd, I must say that I agree here as well.
I vividly remember the first time I tried to solve "shadow over driftwood" by accidentally selling the one quest item and then had a hard time finding it at the right vendor again.
Also sometimes I wonder if I can simply trash stuff or if I have to keep it (at the cost of carry weight, tidyness of bags or just "having to remember I stored it on LV").
Hm, hm. Not the easiest topic - I wouldn't want this to dumb the game down too much, because we already have AAA games for that.