Because otherwise there's a "risk" that we won't know why the proposal has been deleted. Isn't it the whole purpose of this new field?
It's not a big deal, but it seems odd to me that someone puts the time & effort into this, and it might not be used if only optional.
Not to underestimate what I wrote but the patch was an easy one, it does not record the reason just sends the mail with an added field :)
Anyway, it's there so people will probably use it (hopefully); if they don't then we will see.
Anyway, do what you want.
:)
Arnaud.