I've got to say I've never really given much thought to the idea of reader rewards. I'd hope the story was reward enough. The only real answer I can give here is that I am guilty of many promotional stories. I write them regularly. There have been at least six stories told in the Seven Forges world, a short story that precluded BLOOD RED, and a few stories both posted and given away as promotional limited editions over the years. And by short story I normally mean 6-10,000 words or so.
I like giving away stories. They let me cover things that can't be covered in the novels and for me they're like extra seasoning packs to go with the story. They may not add to the main narrative flow, but they certainly add flavor.
here are four examples you can look over at your leisure, if you are so inclined, all related to my SEVEN FORGES series.
FROM THE MOUNDS
THE WOUNDED
SCARS
WHEN KORWA FELL
Keep smiling,
James A. Moore