All you have to do is watch divers in a mixed group of back-mount and side-mount getting ready for a dive to see that the point of side-mount is not simplicity, and that contrast continues underwater (switching regs to keep gas balance, readjusting Al tank position as buoyancy changes, etc). The primary point of side-mount is to get you where back-mount cannot underwater (low overhead restrictions). Secondary points may include being unable to dive back-mount comfortably, wanting redundancy travelling to places where all you can do is rent Al80s, preferring the trim/balance of side-mount, etc. Simplicity, not so much.