While having the rights to a book returned might seem a sign of failure to a writer, or the end of their dreams, it's important to make sure that it's done.
Suppose a writer publishes six moderately-successful books which all go out of print before her seventh becomes an international best-seller. If she's made sure to get the rights to those first six books reverted to her, she can then sell them again—and she's in a position of strength when it comes to negotiating the new deal, so is likely to do well out of it.
At the other end of the scale, consider the new writer who signs with a fledgling independent press. Suppose that independent press can't fund itself adequately and ends up in Administration, as so many do. Without a good reversion clause, all rights that the publisher controls are likely to be considered assets of the company, to be disposed of as the Administrator thinks fit. What chance is there that an Administrator knows his publishing stuff, and will secure the best deal possible for the authors concerned? The same chance that a writer in this situation has of getting her rights back: slim to none. Which means that a book in this situation is, effectively, gone for good.