Recently while doing the JUNOS upgrade on EX series switches where JUNOS image is on external FTP server.
Unfortunaltely one day during change window, we are not able to access the FTP sever for image and at that point of time we have used on existing switch as FTP server and get the image from that switch to destination switch and complete the upgrade without any hassels.
It is new experience for us using switch as FTP server.
Cheers !!
Amit