If you want to move a selection of mails to a specific folder, then that works well even if some of the mails are already in the target folder. However, it fails if the first of the messages is already in the target folder.
In my use case that is quite often the case: I select a whole thread and move it into a folder, but quite often the first of the mails (e.g., my initial message) is already in that folder and then it fails. Of course, one can select all and then unselect offending message, so there is a workaround, but it would be nice if the quick move would act on the others without the need to do that as it is easily overlooked first time and it works well if the messages are ordered differently.