If you can always afford to walk away then this could actually work.
I always give my client's the option of fixed or hourly and really emphasize the benefits of hourly. However I've only been taken up on the hourly offer a few times with recurring clients.
I think it's a great thing to strive for but my advice to freelancer's starting out is not to flat out refuse fixed quote unless you can afford to. Fixed, while not quite as accurate, is still workable once you get a feel for defining requirements on smaller projects. And any larger project can be broken down, so long as you're getting paid for the spec.
Whatever you do don't spend a riculous amount of time specing out a large project without being compensated. I've gotten so many RFPs that are obviously the product of spec work and I'm positive most were unpaid. In fact I guarantee there are forums dedicated to tactics and advice for getting spec work done for free.