Short Stories vs Novel's

Before I've started taking on novels I always thought that to get published you need to have started at the smaller stuff. Sort of like a prerequisite to the big show. However, some of the most recent agented author's I've ran into have never done this step. They got an agent and sold a book without any sort of previous experience.

I know some of the longer known writers, such as Stephen King, wrote short fiction before writing their first novel. Now, I have written close to 20 short stories, and have about another 15 that are in various stages of completion. Some never to be completed. So why do you think that has changed? Do you still think that writing short stories help build your skills for a novel?

I sort of do. My short stories might never get published as my genre tends to lend its self toward longer fiction, but I've used my short stories to help build a character, for example one of my short stories is the discovery of my novel's protagonist's power. It is a decent short story for what it is, if I say so, but I've never gotten it published. I had the same thing with my main antagonist, and both of them have been rejected by numerous magazines. I don't know, but my novel has gotten a lot more positive feedback to it than my short stories.

So what do you think about this? Are you one to write short fiction as well? I still like to, but right now working on a few novel idea's takes all my time. Have you ever written a short story, or do you only write novels? I would like to know.

Oh you can also mention poetry (which I've dabbled in before) or screenwriting (which I've written 3 complete feature scrips and 1 short) or something else. Just curious what you think whether or not Short Stories are a prerequisite to writing a novel, or not so much anymore?