I've seen this done a lot recently--including authors who are selling short stories or collections thereof as ebooks, particularly in between releases of novels in a series--and I think it's a fabulous idea. As you said, it has many writing benefits for the author; I myself have written a few "just for fun" short stories with the characters from my Peter's Angel saga, and it was a great way to experiment with their characters and get back into their "voices" after a break. I released them for free on my blog and HW, and they were well-received.
As you said, these short stories (I've also seen a novella or two with the same idea) can be great for use as free content on your site. I'm also seeing a huge potential market for selling such work, if you've already established an audience. Just yesterday I read "Free Four," a "short story" Veronica Roth, the author of Divergent, released as an ebook in between books 1 & 2 of her trilogy. She took a scene from book 1 and rewrote it from another character's POV. For people who enjoyed the book, it was delightful, and I had no qualms about paying a buck for it, even given how short it was.
http://www.goodreads.com/book/show/13615258-free-four
Similarly, Jordan Smith released a short story collection featuring characters from his as-of-yet unproduced screenplay. A bit of a different concept, but still very nifty: http://www.goodreads.com/book/show/1734 ... er-stories
In other words, I agree--great concept! Go for it. I hope to do it myself one day.
