Ability to create a recurrent task with no due date might be helpful for info purposes. For example, you some list where tasks don't have any particular date, and you do them ASAP. Some of them might be recurrent by nature. So, you can complete them, and the more often you do it, the better, but still no due dates. However, to gather some statistics on which days you did that thing and on which not, you could have tasks like that. For example, "Study some Spanish when you have time" might be a good candidate. Ok, you studied some Spanish, and you mark the task as completed, but you still need that to be in the appropriate list. Later you will see days on which you studied for some purpose. Technically it could be implemented through patterns like "due:every never" or something sounding more human.