This is a great topic and one I have pondered myself.  I usually get to the weathering stage and get antsy to move on to my next project.  It's not uncommon to have two kits going at the same time.  I also have a "Shelf of Shame".  Kits that I have screwed up and waved the surrender flag on them.  I am in the process of salvaging parts from those kits and save them for future use.