If the author can't afford the fees and the book clearly has the potential to be a big seller, we occasionally agree to be paid with a percentage of future earnings. I used to do one on self-editing. One of our editors still does, another editor can do characterization workshops, Ross has a dialogue workshop, etc. Writer's conferences give you the chance to hear or even meet authors, agents, editors, and other people who could be valuable contacts along the long bumpy road to publication. I would never discourage a writer because the manuscript needed a lot of work.
Writing is a lot of work.
Craft can be practiced. Prose can be honed and polished. He had, just that minute, learned the book was on the New York Times bestseller list. At the very end of his luncheon address, he shared with the audience his most rewarding moment as a writer. If you work on a piece of writing until it's as good as you can possibly make it, your worst-case scenario is a tremendous inner satisfaction and the value of everything you've learned during the process of writing or rewriting.