The dialog assumes that someone will probably enter a few lines of Comments when they do the Assign. But many of our users enter in large amounts of analysis text into the Comments field to support their decision to reassign an issue to someone else. The size of the new Assign pop-up dialog field is too small for this purpose.

Also, the pop-up covers the whole Jira window and doesn't allow you to look at other parts of the issue if you need to refresh your memory, copy-and-paste, etc., when entering text into the Comments field.