Someone may join the realtime session while you are editing alone and the editor is switched to Source mode. If we don't merge when switching back to WYSIWYG you may overwrite the changes done by the user that just joined.

Just prevent joining when you're in source mode (write mode). And when more than 1 person has joined, make the source mode readonly.

What's sure is that we need to keep the feature of the source mode when only 1 person is editing at least. I believe it's a useful feature and in 99% of cases users will edit alone and it would be bad to loose this.