I was taught to use Occam's razor whenever in doubt over an octet expansion, being told it never happened when, for example, dative bonds could resolve the issue with a regular octet. This seems to fit what users here say about the recent paradigm shift on "hypervalent" molecules that "it is now known, that for almost all molecules that were described this way the contribution of d orbitals is negligible (<1%).", but could anyone explain exactly why an element wouldn't expand it's octet unless it absolutely has to? What advantage is there to not involving 'd' orbitals over involving them in bonding? Or is it just Occam's razor and I'm misunderstanding this maxim.