In the New York Times today, David Gessner muses on trying to balance the writing life with the working life. He's trying to do both, though teaching creative writing may not be as detrimental to the writing life as say, working as an insurance agent.
Certainly, Sieur Gessner makes an excellent point when he says: "After all, there’s something basically insane about sitting at a desk and talking to yourself all day, and there’s a reason that writers are second only to medical students in instances of hypochondria. In isolation, our minds turn on us pretty quickly."
On the other hand, descending into this sort of insanity is a necessary part of the creative process (so believe I), so if one does not have the luxury of disappearing into one's work, it can be difficult to make progress. As many others have pointed out, writing is the act narcissistic act, the act of a megalomaniac, and neither of those qualities tend to be conducive to being a good employee.
Yet, most writers do need to work just to eat, and to get out into the real world to avoid disappearing into their imaginary worlds, and that's not a bad thing. Believing that one is too focused and tormented to hold down a job is not necessarily the best way to approach one's discipline. And in the end, being a successful writer (published or not) is all about the discipline. It is discipline that turns a scribbler into a writer.
I've had jobs and written at the same time, but my out-put is better when I'm only working on books. Currently, I'm lucky, as writing is my only job. That's about to change, and I'm worried about being able to do big jobs at the same time. But one thing about discipline--it can be cultivated, so right now I'm trying like mad to grow some!