First, Maria: When you say "writing a few doesn't hurt," I have to disagree a little. Basically, writing short stories is an art form that overlaps with but is still separate and distinct from writing novels. There are some authors, lots of them in fact, who can do both. But there are authors like Michael Burstein who are much better at writing short fiction that novels. There are clients of mine like Charlaine Harris and Brandon Sanderson who didn't write short fiction until many years into their careers. Simon Green wrote some short fiction, then turned almost exclusively to novels for many years, and it's only in the past two or three years that he's started to come into his own as a short story writer. If a writer doesn't have the knack for writing short fiction and spends years getting frustrated trying to do them, not actually turning to book-length work that might work because it's not possible to get through the short story apprenticeship, there IS real harm and real hurt to that author to writing short fiction.
Myke says if you don't write short fiction, you lose out on the networking. But people who don't have short story sales still have the ability to network. Go to writer's conferences. Go to World Fantasy or WorldCon. Befriend other writers via writer's groups or meeting at a convention. Some of the mentoring of new writers by established ones may now be easier because social networking allows us to meet in new and different ways. Since the JABberwocky list has as many people with short story sales as without, I don't shut the door to talking to authors who haven't written short fiction.
There's now also a Part 2 on the Hines Survey, click and enjoy here!
I'll disagree just a smidgen with my client here, solely because he puts a big "Busted" tag on the idea that you have to know someone. Just a smidgen. 25% of the authors went out and found someone to publisher a book without a networking connection. So, no, you don't need to know somebody, but when it's a 3-to-1 knowing and not, vs. a 50/50 split on the JABberwocky list for knowing v. not, it's clear that there's enough of a benefit to knowing someone that I wouldn't say "Busted." You don't need to, it doesn't change the odds like if only 10% of the people did it without any networking benefit, but you can clearly better the odds.
But the comments I got from my first post prove why I despair of ever wiping out this "must write short fiction" school of thought. It may reflect some of the insecurity that writers sometimes have. If the writer succeeds via The Path of Short Fiction, it must be because they took The Path, not because of their own skills and talents, and so they really want very much for everyone else to take The Path. And the fully half of all writers who don't take The Path, even they sometimes believe it was in spite of that.