OK, but if a guy comes into my autoshop with a dead battery, and he's talking about how he'd love to race NASCAR someday (but doesn't even know what a muffler is), I'm not about to shoot down his dreams. I'll do the shitty little job I was contracted for, even though I know I'm capable of rebuilding engines, and I don't have the heart to tell this guy that with his severe case of retardation, he'll never even reach 30mph.
I think most mechanics would start laughing and tell them they will do the job, but they will never make NASCAR in this piece of shit.
But, if he leaves, and I start fixing his dead battery, and he calls me fifteen minutes later screaming "WHERE THE FUCK ARE MY ROCKET THRUSTERS?", then that's a dead-beat client. Crazy, insane, asking for the moon, and I'll politely ask him to come pick up his car -- but at this point, I'm lucky if he even pays me for the battery I installed.
This is why you should have a full scope of work written for each client and collect at least 50% of payment up front. "Were rocket boosters agreed to in our scope of work? Nope. Those will cost extra."
Also, automechanic/lawyer analogy is fundamentally fail -- Neither of these professions ever takes on work for revshare, because the final product is not expected to be profitable. You tell a mechanic "make my car work" and you tell a lawyer "keep me out of prison", and these are both established, measurable metrics. If a client asks you to "finish" a site, or "improve" the code, who gets to judge what's "realistic" and what's egregious?
Uh, lawyers work on "revshare" all the time (contingency fees)- when you sue someone, for example, they get a percentage of the amount awarded, usually 1/3. The final product- the outcome of the case- is definitely expected to be profitable in this case, or the lawyer makes nothing, unless he has an hourly rate.
If you are taking a project offering revshare, you shouldn't take the work unless you believe the project will be a success, just as a lawyer wouldn't take a case unless he thinks he can win (or he gets paid anyway by an hourly rate).