I have just solved a problem with that parameter.
I am testing version 4.74 and found that e-mails read from one client with the option "leave a copy of the messages in the server" active could not be read by anyone else because the option "offer..." overlaps the option "leave..." and Mercury serves the message only once.
The option should be off by default or, even better, be removed from options available.
Edited: should be in Mercury Suggestions... Sorry.