The law of unintended consequences is a powerful force. For instance, see this example, pulled from a response on Quora thread “Why do studios re-write scripts after buying them?”

It is to the advantage of subsequent writers to change as much as possible.  I have experienced this from both sides, having had my original scripts rewritten, and having been hired to re-write other writers’ work.  In order to get credit on a movie, a writer must have changed at least 50% of an original screenplay.  Otherwise, he or she may work months or years on a film and get no credit what-so-ever.  The guy who brings the director his coffee gets credit, but the script doctor who wrote 40% of the script does not.  Without credit, the writer is effectively anonymous - no acclaim, no boost to his career, no awards, no residuals on DVD sales, nothing.  As a result, many writers try to  re-invent the script - changing major characters, plot structure, and theme - in order to win credit.

This rule was probably created to protect the original writer by removing the situation wherein someone changes a few passages of dialogue and then gets a co-writing credit. While I fully accept that a screenplay is never sold and made “as is,” but the actual outcome of this rule seems have removed the incentive for making too little/unnecessary changes and replaced it with the incentive to do much, much more.

Perhaps screenwriters are happier with this outcome. I’m not one and I don’t know any so can’t say. I just thought it was interesting take on this rule. 

Another interesting take on the question appears further down the page:

[M]illions of drill owners buy tens of millions of quarter inch drill bits, even though none of them want to own a quarter inch drill bit. What they want are quarter inch holes… Metaphorically, the scripts are like giant drill bits that provide studios with enormous holes to pour tens of millions of dollars into.

(As a side note, one of my fav blogs Asymco has been looking into the economics of Hollywood a bit lately and it’s quite interesting).