Perhaps a quick way to explain to the general public why residuals are an essential part of screenwriter compensation?

To put it relatively briefly, is it accurate to say:

Screenwriters get residual payments because the total value of a script cannot be determined at the time the script is sold to the production company.

(by total value i mean what a movie based on a given script makes upon release, year after year. and that value cannot be known at the time of sale of the script because no one knows how a given movie will perform in the market place until they actually release it; they may have rough ideas at the moment the script is purchased, but those estimates can be way off)

So, people who know a lot about this issue, what do you think?

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[Some background:
I posted the above in a couple of discussion threads at a couple of WGA strike related blogs (Artful Writer, John August - check those sites out for even more background on this issue). They are having long (or at least very involved) discussions re: residuals - as in; do screenwriters deserve them?, how to best explain to the general public why residual payments exist for screenwriters, etc. check it out.]

- Sujewa