Dropbox. sharing-folders. Safety or convenience?

June 10 in the lyceum where I work protection of scientific works. There is little time left, but I still have not seen the work of one of the lyceum students. They drop them to me through dropbox. When debriefing, it turned out the following:
1) The student has sent me a request to share the folder on the wrong mail where I am registered in the dropbox. Dropbox is not registered on this mail at all.

2) When clicking on a link, dropbox offers to register to view files. Of course you do not want to do this, there is already an account.
3) After authorizing and inserting the link into the address bar, I get access to this folder, as if the student was not making mistakes in the mail.
4) Now, when trying to follow the links from the letter, we see: "That invitation has already been accepted."

Obviously, this is a potential vulnerability. Third party access to mail or a banal typo can lead to loss of confidentiality of files.

Do you think the developers did the right thing? Maybe it was worth banning people from another email to access data? Was it worth sacrificing safety at the expense of convenience?

My opinion - let it decide the user himself .