In many Mac applications it doesn't matter if the images are sequestered in the Photos library; you can use "Media>Photos" in a Finder sidebar to access the photos, including albums, moments, etc. Usually only works when going to open or access images from an image app, although it will show up as a browser in like Mail.
You can also turn off the "copy into..." option in Photos prefs so that the images will NOT be copied into the Photos library upon import, but left in their respective folders and referenced there.