"Few" is a red herring; all it takes is one, and in Emacs's case it's more than hypothetical. They were forced to deal with the threat of the project being stalled due to Gosling/Unipress's stake in some code that was reused in the early days. After that, Stallman said never again.
And in general people people in the current era of open source seem to be sloppier/more reckless than before. Ever looked behind the scenes of Wikipedia (or heck, just perused YouTube)? You know how much plagiarism and copyright infringement is involved? Enough to worry about it. (And in the case of media uploaded to Wikipedia, people have to specifically declare that the work is theirs to license or otherwise is free content, but even with those barriers, there are people still willing to do things that jeopardize the project.) Even if only half a dozen people were ever deterred (and the numbers are certainly higher than that), then that alone would be worth it.
