Sure--yes--the student will learn something if they actually wrote a 20-page paper on some given topic. But how are you going to evaluate their ability to compose the 20-page argument?

I would prefer not to be confrontational here, but I am having a hard time imagining that you've deeply considered the pedagogy of how to teach and evaluate students on squishy skills like this.

Knowing a bunch of facts about something is a world apart from structuring a compelling in-depth argument about it.

In the simplest case, where we'll say the exam question was precisely the topic of the 20 page paper, the candidate would be golden. Of course, it's unlikely in a 3 hr. exam that you'll be asked to write a 20 page response; but in edited form, you could definitely produce three cogent pages about some particular aspect of the original paper - if you've done the work. If you truly wrote the 20 page paper, you can surely produce three literate, cogent, responsive and topical pages.