1. The student will do the assignment and hand it in for a mark other than zero.
2. The student will not do the assignment and will keep the zero.The threat of a zero will only extrinsically motivate the students that think they have a chance of doing well in the rest of the course -- but these students are not typically the kinds of students we are really having trouble with.
The students who are the hardest to educate and the hardest to like, are the ones that already get a steady diet of zeroes, and yet they are the ones who need us the most. For these students, they quit long before the zero was even assigned, and now the zero is doubling down on all the reasons why the assignment will never get done. For these kids, there is no such thing as a temporary zero -- zeroes are permanent. If the student had reasons to not do the assignment before, (whether they be real, perceived or excuses) the zero is the nail in the coffin.
Let's think about this for a minute.
It is very likely that dropouts are the kids we have the most trouble with in the real world, and yet they are the ones who get the most zeroes. If giving zeroes helped prepare dropouts for the real world, why is that they are the ones who have the most trouble living in the real world? Alberta does a good enough job giving kids zeroes and failing kids. About 1 in 4 kids don't graduate. Is anyone really prepared to argue that this number is too low?
I find it sadly ironic that people who support assigning zeroes to children do so by claiming it's for the children it hurts the most.