Exclusivity is a big ask. It means turning down other work, limiting your income options, and putting most of your professional time into one client. A retainer at least makes that arrangement fair because it guarantees some income on your end. But some clients want that level of commitment without any financial guarantee in return. They just expect you to be available whenever they need you. It raises the question of whether they fully understand what they are asking for. Have you ever been asked for this, and how did you respond?