Thanks for proposing this, Robin. I think the software could be made to respond (like it does when adding children) so the Default is to RETAIN the parents listed on either profile (if there are conflicting parents the system already requests merging or rejecting them first - something not everybody honors, by the way - but that's another topic!) AND the merger must Tick a box NOT to retain them.
The biggest problem in the merging process is that many people who do merges are either lazy and don't read what's there or they are not very aware of what merging does, so the System Defaults should ALWAYS be biased towards keeping a maximum of information from Both Profiles. What is unnecessary can always be Edited later!