Thats the story of my life, work or otherwise. You've been volunteered, or picked, or whatever because 'x' wasn't around or unavailable to do the task/job/work/ or whatever aspect its about. All the work you put in isn't worth a damn because people think you'll be there to pick up the pieces if their 'choice' lets them down or leaves.
I wonder what would happen if some people/departments find out that I'm not going to be that. I am getting a tad fed up of being 'the backup'.