Most 'paid' assignments don't get made into films. I'm sure somewhere on the net you will find a statistic of 90-10 or 98-2 (more likely) ratio to prove it. Chance of a 'no-money-upfront' deal making it through is set in fairytale land. Sure it happens, though I don't know anyone it has happened to, but it is the same thing as buying a lottery. In fact it is worse because you've got to work for 3-6 months before you have a shot at the dream as opposed to just forking out a couple of bucks.
My simple logic is: if a producer can't raise money for a script he's probably just fishing anyway. Basically the producer's plan is to take a writers investment and try to build his fortune on it. Doesn't sound like a good business deal to me.
I get a counter logic from desperately needy writers: "What if it was a reputable, verifiable person asking, should I do it?"
Writers are an insecure lot and easily buy into false dreams. But they've got to stop to think about what these producer credentials are worth. I mean, if they can't raise money for the most important component of a film they couldn't be worth much. Remember without a script a producer would have nothing.
The other fact to consider: A writers job is done when he hands in the script. So basically the writer is the only one to have delivered on his commitment. Everyone else can just walk away from there as they have nothing invested in it anyway.