Where are you getting 6 / 2 from? Eddie tells us Earl can do anything so it's 5.5 / 2.5.
Because 9 is so specialist one of your back subs is invariably a 9 leaving one sub to cover all the other back positions in a 6 / 2. Fair point on versatility in the backs but few players can play multiple positions equally well. And there's always the risk of multiple outside back injuries - it happens......e.g Eng v Wal Eng in 2015 RWC when Amos, Scot Williams and Liam Williams were all forced off and they ended up with a SH on the wing.....and we all know how that ended
SA went the bomb squad route because it suited their strengths and resultant tactics. But if you're looking at a more balanced game then 6 / 2 is a bigger risk.