I think guest lectures are helpful: they broaden the perspectives and expertise available in a given course. In general, they happen as the result of a kind of “gift economy”: you agree to give a guest presentation in a colleague’s course without expecting any kind of reward, even a return visit from that colleague. Gift economies can work quite well–sometimes more efficiently than market economies. But there is no norm in academia of offering to give guest lectures. Instead, you have to ask someone to be a guest in your class, and that can be awkward. It’s a gift economy in which the recipient initiates the arrangement: not a recipe for success.
Thus, if guest lecturing is beneficial, we should switch from a flawed gift economy to some kind of exchange system. Professors should earn credit for giving guest lectures. I am not sure I would define the credit as a right to receive a guest lecture in one’s own course, because there might be no one available to provide appropriate material. Instead, I would identify some modest good that is in short supply and offer it to professors who amass sufficient credits for guest-lecturing.