In this case, "if you're not doing it full time, if you're not fully committed, you always have an easy out. By quitting, and officially dedicating yourself to your new venture, you've given yourself a big incentive to give it your all. It's like the Greeks who burned their ships on the shore" (or something along those lines).
If the norm was to do a startup on the side whilst remaining in full time employment, then I'd say it's pretty likely 37singles would do a post about how you should have the balls to quit your job and how you're doing yourself a disservice etc etc
In this case, the post reflects 37signals' experience building their product business as a "side project" while still doing consulting work as a "day job."
When they focus more on "Here's what we did and why we think it worked for us" instead of "Here's what you're doing and why it's dumb" then their posts are much more informative.
I believe in this case armies were showing that they were fully committed to your enemies. It was a demoralizing act to defenders.
The same might be said of startups. Quitting can signal to others (investors/potential employees/customers) that you are serious. However, I feel like this thread has taken an introspective slant. I'd caution anyone quitting who is trying to signal to themselves.
Also see http://en.wikipedia.org/wiki/Point_of_no_return#Related_expr...
Fear can be a powerful motivator. Of course, if things go wrong, you end up murdered by massive armies of Indians. Or something.
"If you treat it like a hobby, it will be a hobby. If you treat it like a business, it can be a business."