Basically, here is the problem I am seeing regarding the 21 days.
Let's take the example of a member who lives very far from Ottawa and would have to travel for 24 hours to be present when his spouse gave birth. He could completely miss the birth. So it can be expected for them to want to remain with their spouse as of the 36th week of pregnancy.
If, by misfortune, the 36th week of pregnancy happened to fall within a House sitting period, the 21 days could be used to cover the period when the member is staying at home, but he will be left with no days for any other leave reasons. Let's take the case of a member who has already had to miss two weeks of sittings for other reasons that are not covered, such as to attend his father's or mother's funeral. If he wanted to take another leave to stay with his wife who is close to giving birth, the 21 days may not be enough.
It is more in that kind of a situation that this could happen. It may not have happened in the past, but it could happen.