I haven't had a use case for that but it sounds useful.
Well, my use case involves any kind of get request. If you're using s:link or h:outputlink with conversation propagation you run the risk of being able to have 2 windows open linked to the same conversation which can cause problems in some cases.
In my particular case I'm creating a list detail pattern example with seam. I don't want to store the list page's properties in the session because I want my user to be able to have the list page in multiple windows with independent search params and page numbers and such.
I'm using a get request (s:link) to go from the list page to the details page because I would like the user to have the option to do an "Open in new window".
Once the user is done with the details page I navigate them back to the list remembering the page and search parameters used when I navigated from the list page before. However, if the user used "Open in new window" or something like that to navigate from the list page to the details page then I have 2 browser windows linked to the same conversation which share search parameters and paging information.
So what I'd like to do is when the user navigates to the details page "clone" the current conversation no matter what that way weather they navigated to the details page using "Open in new window" or whatever all instances of the list page will remain independent.