A dialog for File->Merge

[MikeJ]

In my mind the selections in the file to be merged in should be ignored

We can't ignore it. The current operations for Merge command will replace any current selection in the scene that you was working. So, it has been working this way for long time and we don't want change any workflow that somebody else can be using. (I think compatibility and extra control about selection)

Thank you for this clarification.

So the current workflow that people have been using forever is that when you are working in a current file and you want to merge something in from a different file you have to save the current file and close it, open the file you are merging in from, make whatever selections you need to make in that file such that the selections are correct when you merge it into the current file, save the file, close the file, reopen the original file, make whatever selections you need to make in that original file and then perform a merge?