Hi Torsten,
it was a suggestion in Kontakt 5.1 loading time - Cantabile Community (cantabilesoftware.com), but that was 2017, so that might be outdated.
If I understand well Brad is saying it is instanced once, so no duplications: Master Rack… how freaking stupid am I? - Cantabile - Cantabile Community (cantabilesoftware.com)
My background rack is not so complicated yet, so for me this could work, however I would rather follow a best practice now and avoid problems later.