There's an interesting piece in The Chronicle by Karen Kelsky filled with advice about grad school, and what to do before, during, and after. It includes this:
Go on the market while A.B.D. because you want to make your worst mistakes while you still have a year of financial support from your home department. Most people who prevail on the market need at least two years to do so.
I'm inclined to agree with her on this point. Although these days, many advise against going on the market ABD, given the stiff competition from a backlog of PhDs, if you go in knowing that you're not likely to succeed, you can use your failures as learning experiences. I was lucky and got a post-doc my first year (while ABD), but I definitely learned from my many failures that first (and second) year, and it (eventually) helped me. My third year, I actually got an interview with a school that passed me up the first two years I applied, so I either improved greatly, or was not so memorably bad that it hurt me.
On the other hand, going on the market ABD means you must survive another year of the ultimate experience in grueling horror.