Short stories can contribute a lot to a writer's success. Back in the early days of Science Fiction, pulp magazines were the lifeblood of such famous authors as Heinlein and Asimov. Today, most of those big periodicals are long gone, yet releasing short works is still a valuable part of marketing and promotion. Therefore, many up-and-coming writers are still building their careers on their smaller works.
A lot of writers have a knack for cranking out little bits and pieces, and find putting together a full-length novel to be a challenge. I, however, have found it to be the other way around, as I am more adept at crafting a long, complicated tale with various sub-plots and side stories. Many a time I have sat down to write a "short" story and ended up with the first chapter of another book.
Jack London ran into this same dilemma, when he began writing a short story that ballooned into "Call of the Wild," so it's nothing new. It is a natural inclination for a writer to expound upon his ideas, and tell "the rest of the story." It is sometimes hard to sum things up in only a few thousand words.
Readers, however, are not always looking for a lengthy read, and in this era of mass media and instant gratification, more and more people are looking for quick fiction, something they can digest in one brief sitting. You'd think this would create a greater demand for short fiction, but there is still limited space on a magazine's pages, even if it's printed online. Therefore, the markets can still be tough to crack.
I have had limited success with marketing short fiction, though I have managed to get a few stories into print anthologies during the last year (Patented DNA and Haunted: An Anthology of the Supernatural just to name a couple). I haven't seriously tackled the short fiction market as I most likely should have, and I hope to remedy that in the near future. Once I've completed the Fantasy Western I'm writing, I will begin work on a series of short pieces, specifically designed for various online zines. Whether they'll find much success is never certain, but if I bombard the marketplace with enough quality work, somebody somewhere will bite.