Program X is GPL'd. You want to modify it but don't want to release your modifications. Slightly modify the source of X so that it can dynamically load foreign executable code. Have it load your proprietary additions.
Release the source of your Program X fork as GPL, but do not reveal the source of your proprietary additions.
This is why I think the ``content vs. code'' paradigm is stupid. According to the FSF, I can write a book and do whatever I want with it--including using it in a piece of software--but for software itself I have to use one of x licenses or I'm being unethical.
Literary works are just text. Code is just text. Yet if I want to include that code in GNU software, it has to be under the GPL. What about the text in the code?
puts("Neither a borrower nor a lender be; For loan oft loses both itself and friend, and borrowing dulls the edge of husbandry");
What now? Am I supposed to tokenize all my ``content''?