So you say you don't agree when in your example you say the project was poorly implemented, poorly documented, and unmaintainable? This is exactly why a rewrite is justified. I'm also saying that if it is properly implemented, well documented, and maintainable then the arguments for a rewrite are rather thin - just refine what you've got.
The article Ovid posted was essentially saying, "A rewrite is bad when the code a) works, b) is bug fixed, c) well documented, and d) the over-all design is sound." If the project is missing too many of those points, a rewrite is justified. How many of those points and to what degree are need to justify a rewrite are, like you said, why we get paid the big bucks =)