When a new assignment is created with a maximum grade of "no grade", it does not appear in the gradebook. However, when you change this item to have a value for maximum grade it does appear as a gradebook item, and it continues to be a gradebook item if you re-assign it to no grade.
The correct behavior is to have it be a gradebook item even if "no grade" is set upon creation. It will simply play no role in a computation. Otherwise it will (and has) caused teachers much confusion when the believe that some of their assignments have been lost in the gradebook.
It is not uncommon to have an assignment listed as no-grade to be a place holder until the teacher adjusts that assignment the number of points desired. So the behavior suggested is important. But in any case, the current behavior is inconsistent regardless of ones view of whether assignments with not grade should be listed.