There's an Edutopia article cycling around which argues against allowing retakes on assessments. The author builds a case focused on motivation, mental health, and teacher effectivness.
Instead of doing a large breakdown here, I annotated the original article with my pushback in context.
On a semi-related note, If you don't have the hypothes.is extension installed, you should grab it and leave replies on the original article. Comments are also welcome here.