I like doing the challenges (they run through March, and then again all through September), because they give me a chance to experiment a lot. It's not like a novel where you have to stick with a character for 100,000 words. A short is something where, when you reach the end, you can walk away from a failed experiment without a great deal of time and anguish invested. By experiment, I mean exploring characters I would not want to use in a novel, or situations I don't much want to write about but are cool to look at once in a while, or settings that don't involve reams of worldbuilding. An SSIAW story is, of necessity, written at full speed, and I try to get in two or three in a week, so go figure on the quality of the finished product. Some of them haven't been bad, though. "An Infinity of Moments," which recently sold to On Spec, was the product of an SSIAW. So was "Message in the Dust," which will appear soon in Science Fiction Trails. But anyone going in who just can't conceive the notion of sharing a rough draft, warts-and-all story with the world is not cut out for SSIAW.
The point is to write. Just get in and write. Finish something. Meet a deadline. It's an exercise in discipline as much as it is in writing, so people who pooh-pooh the notion of trying to churn out anything of value in a week should consider that angle. There are many writers who sit around and imagine their stories to death without ever writing a word; there are others who actually get it on paper, then tinker and tweak and fuss and fiddle with it ad infinitum, never actually subbing it anywhere, either for critique or for publication. These are not real writers. These are people who call themselves writers but don't actually try to make the leap into being recognized as such by anyone but their loving kinfolk.
Back to experimenting. I am a novelist at heart. I suppose that's why I can toss of 20,000 words worth of short stories in a week and leave them moldering in the drawer for years. Only the really good ones grab me enough to want to devote the considerable time required to workshop them, polish them, and shop them around. Short story rejections break your heart fully as much as novel rejections; worse, they come back faster and generally more often. Seven times the misery. Heh. However, they are an excellent venue for trying things out: stories from the antagonist's viewpoint, stories with unsympathetic protags, stories with ultra-weird protags, stories with fantastical settings. I intent to use the March SSIAW coming up at OWWW to explore a little more than I usually do, praying all the while the word lists give my muse a creative jolt in the right direction. If not, I suppose I will settle for writing anything, as usual. But I hope for originality.
Don't we all?